-SPOILER WARNING-

Currently the most prevalent theory, and why it's wrong:

The most common theory on the internet is that the two shamans are bad and the girl in white is good.

There are a few details in the movie that suggest that the above conclusion is false:

1) The forest shaman, on the day after his interrupted ritual on the dead guy in the car, hurries to the car in an agitated manner. When he sees that the dead body isn't there, he is genuinely afraid and anxious.

Why would he be afraid in this manner if he had achieved his intended goal of resurrecting the corpse (assuming he's the antagonist)?

2) When Jong-goo's deputy is telling Jong-goo and the deacon the story about how the shaman uses the photos to infect/possess the dead, he is very obviously (both to the audience and the characters) in a non-lucid state. It appears as though he is possessed when he is relating this story.

If the deputy is indeed a trustworthy character why does he kill his family like the rest of the possessed?

3) If the invited city shaman, Il-Gwang, had evil intentions, why was he invited all the way from the city? Why did he want a large sum of money to perform the death hex if his motive was to hurt the family? Couldn't he just kill them and take their money?

It is not surprising that a lot of people fell for the wrong interpretation. The movie ingeniously tries to sway the audience and lead them to false conclusions just as the evil spirit deceives and misdirects the people of the village. In oriental superstition evil spirits are notorious for their deceptive ability. They always choose deception to coax their victims rather than using a straightforward approach.

Now I'll go into an analysis of all of the important hints and events in the movie and what they mean. If you don't care about the hints and want to know exactly what happened in reality, then go to the end to see the summary of important events.

The Woman with the Trinkets

The woman in white seems to always wear or carry around trinkets. The trinkets and apparel she is wearing are from people who are possessed. Late in the movie you see her wearing the cardigan of an infected girl and you see on the ground near her there is Hyo-jin's hair clip.

A far less obvious trinket is apparent when you see her for the first time in the movie. During our first encounter with her, she is wearing Park Chun-Bae's jacket. You see the jacket again later being worn by Park Chun-Bae when he is dead in the car. When the girl was wearing the jacket Park Chun-Bae was likely already possessed.

An Officer Possessed

Jong-goo's deputy, Oh Sung-bok, was the first to come up with the speculation that the rashes could not have been caused by mushrooms. He later does a complete 180 on this matter and starts dissuading Jong-goo from investigating the illness further. It is my opinion that Oh Sung-bok got possessed and this is what caused his 180. After his possession, all of the information that he provides is misinformation.

The possession took place on the night the two officers (Jong-goo and Oh Sung-bok) are visited by the naked possessed girl at the police station. The evil spirit attempted to posses both officers that night in their sleep, but it could not posses Jong-goo because Jong-goo was too pure or strong willed.

The morning after that night is the first time we see Jong-goo wake up in an agitated state, waking from a nightmare. In his sleep he says "Stop goddam it!", and he wakes up saying "Stop it you dirty slut! You whore!" This suggests to me that Jong-goo resisted his possession. Oh Sung-bok that same night was unable to resist possession leading to his 180 opinion change about the rashes and the forest shaman and also leading to his future misdirection.

Possessed Oh Sung-bok doesn't want Jong-goo to investigate the forest shaman because the forest shaman might reveal the truth about himself (forest shaman never does reveal the truth about himself because he believes that everyone has already made up their minds and the truth won't matter).

The Biggest Misdirection

The biggest misdirection that possessed Oh Sung-bok is part of is him leading everyone to believe that the forest shaman was using the photos in his hut to possess people. Look at how Oh Sung-bok behaves after seeing the photos and during the car trip back, when he claims the forest shaman to be a criminal. It is obvious that he is not speaking, but the evil spirit that possessed him is speaking. In the car Jong-goo even asks Oh Sung-bok "What's wrong with you? Did you get bit?" and says "You're scaring me man. Snap out of it", this is because Oh Sung-bok was behaving so oddly.

The True Motive of the Photos

The photos taken by the shamans are not used with malicious intent. Instead, they use the photos to perform rituals so that the dead cannot become possessed. The biggest clue for this comes when the Japanese forest shaman takes a photo of the dead Park Chun-Bae in his car. He surrounds the car with candles and takes the photo home to perform his ritual. The forest shaman's ritual is interrupted, and the next day we see the wounded forest shaman run to the car. He is visibly upset when he sees that Park Chun-Bae's dead body is not there. This confirms forest shaman's suspicion that his ritual failed and Park Chun-Bae dead body is possessed.

The protection of dead bodies against possession is a common theme in oriental superstition. You can see this theme in the movie Rigor Mortis (2013) as well. The rituals send away the souls from the bodies so that the souls cannot become possessed.

Park Chun-Bae's body is the first dead body to be resurrected out of all the deaths in the village. This is because the previous dead bodies have had successful soul sending rituals done on them by the forest shaman.

The Possession of Hyo-jin

Hyo-jin is first contacted by the evil spirit on the night she delivers fresh clothes to her father at the police station. When she leaves the station she drops her hairclip. Jong-goo discovers the clip, but when he is about to pick it up he interrupts himself recalling the naked girl from the prior night. We never see him pick up the clip. This clip ends up in the hands of the woman in white.

Hyo-jin initially resists full possession. We see her recollecting a nightmare "someone keeps banging on the door, trying to get in". I believe that her resistance is facilitated by the forest shaman who uses her shoe to help in a resistance ritual. The next day, Jong-goo, deputy and deacon visit forest shaman. Possessed deputy Oh Sung-bok steals the shoe from forest shaman's place. When Jong-goo returns home with the shoe, we see that Hyo-jin is now fully possessed.

Il-Gwang and the case of Mistaken Identity

We see that a shaman is invited from the city in order to help Hyo-jin. His name is Il-Gwang. During Il-Gwang's investigation of Hyo-jin, he inquires about someone Jong-goo met, someone he was not supposed to meet. Jong-goo mistakenly thinks that Il-Gwang is referring to forest shaman during this inquiry, but Il-Gwang was actually referring to the woman in white.

Due to this case of mistaken identity, Il-Gwang mistakenly believes that forest shaman is the evil spirit. So, Il-Gwang attempts to kill forest shaman with his death hex.

Two Failed Rituals

The pain inflicted by the ongoing death hex causes forest shaman to stop his ritual to protect Park Chun-Bae's body against possession. Jong-goo interrupts Il-Gwang's death hex ritual, and as a result forest shaman has a chance to live another day.

The Death and Possession of Forest Shaman

After forest shaman almost dies to the death hex and crawls into bed, you can see a brief glimpse of the woman in white ominously wandering outside. This is an important piece of allusion. The woman in white wants forest shaman to die so that she can possess him.

The next day when Jong-goo and gang chase forest shaman, you can see that forest shaman is obviously very distressed. If forest shaman were really a spirit, or a ghost, or a demon, he wouldn't be scared in this manner because people cannot kill ghosts.

Forest shaman is finally killed by the car and thrown off the ridge. There is no shaman around to take a picture of forest shaman's dead body and perform a final passage ritual, so forest shaman's body is free to become possessed. Indeed, the forest shaman that we see in the final act is the possessed dead body of forest shaman.

The Grand Final Meeting Between Deacon and the Possessed Dead Body of Forest Shaman

First off all, the set-up for the meeting; why does the deacon decide to visit the forest shaman's home in the wilderness? The deacon is lying in a hospital bed with bandaged cheek and looking at the TV. The mushroom illness is spreading throughout the village and making people deranged, according to news reports. Next, the deacon visits Oh Sung-bok's house. Oh Sung-bok by now had already killed his family confirming that he had been possessed.

The look in the deacon's eyes as he looks at On Sung-bok is a call back to the look in his eyes much earlier in the movie. When On Sung-bok, Jong-goo and the deacon are leaving forest shaman's home by car after their first visit, you see several times the camera focuses on the deacon as he listens intently to possessed On Sung-bok's intricately weaved false narrative.

It is important to note that when On Sung-bok et al. are preparing to visit forest shaman for the first time we see him visibly wearing a cross. On Sung-bok is still wearing the cross when he is on the floor, after he has killed his family.

Watching possessed On Sung-bok puts doubt into the mind of the young deacon. He must visit the house of the forest shaman not only to confirm his suspicion that the forest shaman is responsible for the continued possessions in the village, but also to reaffirm his faith. Under this light, the dialogue between the deacon and possessed forest shaman means a lot more.

Deacon, desperate to reaffirm his own belief, asks the possessed forest shaman what his true form is.

Possessed forest shaman responds, what do you think is my true form?

Deacon says, the devil, you're the devil. - The word choice is important here. He said, "You're the devil", not I think you're the devil, not I believe you are the devil, not I suspect you are the devil. There is no doubt in his tone.

Deacon then presses possessed forest shaman for a response, what is your true form? - Deacon must get confirmation about his beliefs.

Possessed forest shaman says: "You have already said it, I'm the devil." - "Isn't that right? You are already certain I'm the devil. That's why you came here... carrying that sickle" - "My words, whatever I say, won't change your mind."

Deacon refutes, "No, that is not true." - he is more hesitant now, more doubtful about his belief.

To this possessed forest shaman responds: "There is no doubt." - doubt here has a double meaning. There is no doubt that my claims are correct, there is no doubt in your mind about your own beliefs. "You came here to confirm your suspicions about me. " - confirmation bias.

Now, decon gets upset and emotional, he shouts "No! If you say you are not the devil and reveal your true form to me - I'll leave you alone." - This is the first time that deacon challenges his own beliefs. This is the first time he allows the idea that forest shaman might not be the devil to enter into his mind. He could have questioned the false narrative prior to the death of the forest shaman, the forest shaman could have lived, but by the time he began questioning the false narrative it was already too late. Now, the forest shaman is dead and possessed.

Furthermore, the proof that the deacon requires is a revelation of the true form. When forest shaman was human, he would have no way of proving that he was not the devil, because his true form was the human form. This line of reasoning has similarities to the Salem Witch trials.

The Mirrored Interrogation

Earlier in the movie we saw deacon and Jong-goo are interrogating the real, unpossessed forest shaman. The interrogation then went in a similar vein to the deacon's solo interrogation in the cave, but Jong-goo had made up his mind much more steadfastly about the intentions of the forest shaman because he had fully adopted the false narrative.

Jong-goo asks forest shaman why he was there.

To this forest shaman hesitates and responds, "To travel."

Jong-goo then says "Tell him to confess, or I'll throw him in jail", but deacon translates this to "Tell us the truth."

Forest shaman responds: "Even if I told you, you would not believe me."

After this Jong-goo starts cursing, making threats and repeating the false narrative.

Just as the possessed forest shaman claims in the cave, they had already made up their mind and were only looking for confirmation of their beliefs. Nothing he could have said would have swayed their mind, because they had already bought in to the deceit of the evil spirit.

The Liar, the Truthsayer and the Nihilist

The girl in white is the liar along with all she has possessed, including the deputy, Oh Sung-bok.

Il-Gwang is the truthsayer. He has no incentive to lie. He has no ulterior motive besides money.

The forest shaman is the nihilist. He believes that the truth won't matter because nobody will believe him.

All of the above I have a low degree of uncertainty about. The following is something I'm unsure about as it requires a lot of assumptions:

The Girl's Trap

The girl in white, or the evil spirit, says that she has laid a trap for Il-Gwang and that all Jong-goo needs to do is wait for the trap to be triggered.

The evil spirit had a lot of incentive to kill Il-Gwang. Il-Gwang is the only shaman remaining who can take photos of the dead in order to perform the sending ritual, thereby making the dead impervious to possession and resurrection.

When Il-Gwang was trying to escape the village by car, the evil spirit sent him visions so that he would return and get caught in her trap. How exactly the trap works is unclear, but it is clear that the flowers in the archway is the trap.

You will notice that when Il-Gwang first comes to town and goes to Jong-goo's house, he very carefully inspects the archway and the hanging bushes before entering through the archway. At the end of the movie when Il-Gwang enters the archway, he again hesitates before entering.

It is clear that Jong-goo triggers and breaks the trap when he enters to archway to find his murdered family. This is why the evil spirit is upset when she couldn't stop Jong-goo from leaving her to go back to his house. The trap was intended for Il-Gwang, but Jong-goo's triggering of the trap foiled her plans.

There are a few possibilities as to how the trap might work:

Current best hypothesis about the trap: The flower trap is a ward against Il-Gwang. This is what stops him from passing the girl earlier and induces blood/regurgitation. Since he cannot pass, he cannot render assistance to the family. If Jong-goo passes the trap, the trap breaks; this is why Il-Gwang pleads with Jong-goo to go to the house.

The evil spirit/girl trying to keep Jong-goo with her has a dual purpose: 1) It stops Jong-goo from breaking the trap, 2) She is trying to keep Jong-goo with her in order to possess him. The possession requires time and requires that the victim is fully convinced/persuaded/deceived by the evil spirit. As she lies and convinces Jong-goo of her story, she draws closer and closer to Jong-goo and finally, at the later stages of her possession attempt, she is able to touch Jong-goo's hand. Jong-goo sees his daughter's hairpin and starts having doubts about her lies, this is when he is able to break free from the possession attempt.

Other possibilities:

Possibility 1) If someone goes past the flowers, the flowers wither. The person who moves past the flowers become impure or a sinner, and thereby they are vulnerable to being interacted with/possessed by the evil spirit. Jong-goo had already sinned when he killed forest shaman, this is why the evil spirit could touch his hand. When Jong-goo passes the trap, it has no effect on him because he was already a sinner, but him passing triggers the trap and as a result it cannot be triggered again when Il-Gwang approaches the trap later.

Possibility 2) We see very early in the movie during the investigation of the first murder scene, Jong-goo notices withered flowers at the pig farmer Hueng-guk's house. One room in the farm had a nest-like bed which we know is used in death hex rituals where a possessed person sleeps in the bed while the shaman attempts to perform a ritual to kill the perceived evil spirit.

Based on the level of infection in the Hueng-guk's face, we know that the he had been possessed for some time. Cho, the ginseng farmer, was possibly a shaman who was called to exorcise Hueng-guk, but because Cho triggered the flower trap, the ritual backfired (we know from Il-Gwang's words earlier that the hex is prone to backfiring). The backfire caused Hueng-guk's possession to accelerate and as a result he kills Cho, and later his family.

Possibility 3) The trap causes the possession to spread. The evil spirit wanted to infect Il-Gwang so that he could carry it out of the village and infect others beyond the village. She is upset when Jong-goo unintentionally ruins her plan by triggering the trap because all of the village is already almost dead and she has nobody else to infect/possess.

Possibility 4) The evil spirit was lying about the trap to keep Jong-goo with her for longer so that she could possess him. The possession possibly takes some time to complete. If the rooster crowed 3 times then she would have enough time to possess him. Watch how she draws closer and closer to him and touches him near the end. She fails the possession when he sees his daughter's hair clip and he starts doubting her.

Possibility 5) The flower in the archway helps the evil spirit create a barrier so that the shaman can't enter. This is why she is able to make him bleed/vomit earlier when he tries to pass. When Jong-goo passes the barrier, he breaks the protective barrier allowing the shaman to enter.

Summary of important events:

1) Jong-goo and deputy go to investigate first murder.

2) That night they get visit from naked possessed lady. Later that night the evil spirit attempts to possess both of them in their sleep. Jong-goo's possession fails, but deputy gets possessed. Deputy will constantly lie and misdirect from now on.

3) On the night of investigation of second family death, Jong-goo's daughter, Hyo-jin, comes to the police station to drop off clothes and on the way back she gets possessed.

4) Jong-goo meets and interacts with girl in white/evil spirit at the site of second family death. She lies about the forest shaman.

5) Hyo-jin starts showing signs of possession compelling Jong-goo to take action

6) Jong-goo, possessed deputy and a deacon go to visit forest shaman in the woods.

7) At shaman's house they discover a ritual room and a room with lots of photos/belongings.

8) On the way back to the village, possessed deputy lies and tells deacon and Jong-goo that the forest shaman was using the pictures to possess people. This is a lie. Infact, he was using them to perform protection and soul sending rituals so that the dead couldn't become possessed.

9) Hyo-jin's possession symptoms increase because the deputy recovered her shoe from forest shaman's house. Forest shaman was using the shoe to cast a protection ritual.

10) Jong-goo and deacon visit forest shaman again and interrogate him. Forest shaman says that they wouldn't believe the truth.

11) Second shaman, Il-Gwang, comes from the city. He asks Jong-goo about someone Jong-goo had met who he was not supposed to meet. Jong-goo mistakenly says the forest shaman; infact the person he was not supposed to meet was the girl in white.

12) Il-Gwang casts a death hex on forest shaman. Forest shaman is simultaneously casting a final passage ritual on the dead guy in the car, Park Chun-Bae. Both these rituals are interrupted. Forest Shaman does not die from the death hex and Park Chun-Bae's dead body is vulnerable to possession.

13) The next day, Jong-goo and gang visit forest shaman's house in the woods yet again. They chase and eventually kill forest shaman.

14) Il-Gwang sees the evil spirit, but she can't possess him. Instead, blood spurts out of his nose and he vomits. Il-Gwang attempts to escape the village, but the evil spirit won't let him leave.

15) The deacon sees the continued deaths in spite of the forest shaman's death and starts to have doubts. He visits the forest shaman to get some answers. Forest shaman is now possessed and he repeats what alive forest shaman said previously, the truth doesn't matter because nobody will believe him. Deacon vehemently disagrees, but now it's too late to ask for the real truth because forest shaman is already dead and possessed.

16) Jong-goo meets the evil spirit and gets a phone call from Il-Gwang. Il-Gwang tells the truth about the mistaken identity of the forest shaman and tells Jong-goo not to get tempted by the spirit. The evil spirit constantly lies and tries to keep Jong-goo with her. She has a dual motive for keeping him: 1) so that he doesn't trigger the flower trap that is meant for Il-Gwang (who will enter later) 2) (possibility) She needs time to possess Jong-goo and Jong-goo has to fully believe her before she can possess him.

17) Jong-goo leaves the evil spirit and triggers the flower trap. The evil spirit is upset because she couldn't accomplish either of her goals by failing to keep Jong-goo with her.

18) Il-Gwang comes to Jong-goo's house after everyone is dead. He can pass the archeway because the flower trap is dead, thanks to Jong-goo. He takes photos of the family so that he can cast soul passage ritual, so that the dead bodies don't become possessed.