One of the reasons why this is such a landmark film is that Hitchcock bends the definitions of good/evil and hero/villain. Here we have 3 main characters: Alisha (Ingrid Bergman), Devlin (Cary Grant) and Sebastian (Claude Rains). Up until the late 1940s the standard romance thriller had a clear distinction between good & evil. But I think this is one of the first films that broke that mold.

If we accept Ingrid's character as the protagonist, since Hitchcock gives her the most screen time, the most closeups & lines of dialogue, and especially the memorable POV shots where the audience is inside her head, then the question of villain boils down to Claude Rains or Cary Grant.

On the surface Claude Rains is the first pick since he's with the Nazis. But I think this is a clever deception by Hitch. As we learn, Claude's character is genuinely in love with Ingrid's character, he never doubts her, and he never treats her harshly. Only toward the end after she betrays him does he become her antagonist.

In sharp contrast, Cary Grant's character treats her with cynicism, taunting and even several moments of literal violence (punching her in the car when she tries to kick him out, or their first forceful kiss on the mountain). I think the real protagonist/antagonist scenario is done by Ingrid/Cary. And if you think about it, Cary Grant's character could've easily stopped the whole mess in the beginning by simply telling her "don't take the job. Let's get out of here" which is what she was begging him to say. Instead, almost like Mephistopheles in Faust, he becomes a sinister character who leads the protagonist into a series of mistakes that bring about the protagonist's downfall.

Cinematically, Hitchcock sends us clear messages that this is his intent, such as the introduction to Cary Grant's character which is all done... remember...? with the character completely drenched in menacing shadows. At the party he's the only character whose face we can't see, who doesn't utter a word but just sits there like a puppet master while Ingrid's character tries to talk to him. We should've guessed it from that opening scene. But we're fooled by the Nazi angle into thinking Cary plays the good guy. I'm sure it fooled a lot of audiences at the time, but it just goes to show how Hitchcock was so far ahead of his time. Sympathizing with a "Nazi" while portraying the American as a villain? Even 100 years later you can't get away with it. But Hitch did. He did it by cutting through the BS of labels and stereotypes and instead showing us how real humans act, on all teams.