@Markoff said:

What a horrible punishment, the guy visit his mom, jog around hometown and everyone is fine with it, I thought you can leave prison only under special circumstances as funeral or dying direct relative and not just getting leave few times a month as movie imply.

I agree that the film isn't very clear about the circumstances for the prison permits. I think that one of the special circumstances was:

Excerpt from - translated (Spanish to English) - "I'm sure my husband would have tried to talk to his own killers" - Maixabel Lasa decided to meet one of the ETA assassins who made her a widow:

Lasa was part of a pilot program launched by the Socialist government in 2010, to get jailed ETA members to meet relatives of their victims face to face. It was meant as a way to start healing wounds in preparation for the terrorists' expected announcement of a permanent ceasefire, which eventually took place in October 2011.

The first four meetings were held in May 2011; there would be seven more later on. At the time, Lasa was the only victim who agreed to meet with one of the actual perpetrators of the attack on her husband. It was he who had requested to meet her. Additionally, the Prisons Bureau had asked her agency for help with the program, to see if she could find victims who might be interested in participating.

Every country has their own prison system rules.

Here are some articles - translated (Spanish to English) - about Spain.

... How do prison permits work?

... Excerpt from Reducing Discretion in the Administration of Prison Leave: In Search of Legitimacy:

In Spain, the penitentiary law requires three ‘objective’ elements to be satisfied prior to an application for a prison leave: ¼ time of the sentence served, medium security classification and good conduct.

Here is an article (2015) - translated (Spanish to English) - about one of the ETA prisoners, Luis Carrasco.

... Six days of leave for the murderer of Juan María Jáuregui for "good behavior"

Sidenotes:

In Spain it is still very taboo, politicized and controversial when discussing the ETA. Convicted members currently in Spanish and French prisons are considered to be political prisoners.

... ETA (separatist group)

so after quarter of sentence can good behaving mass murderers in Spain enjoy time outside? insane

the movie shows him alone just visiting mother, not going outside only to see the victim

@Markoff said:

so after quarter of sentence can good behaving mass murderers in Spain enjoy time outside? insane

I don't think that it's that easy, but with good lawyers it could be possible.

the movie shows him alone just visiting mother, not going outside only to see the victim

It is possible that as part of the pilot program in 2011, he also got a prison permit to visit his mother. As I've already posted, in 2015 he also got a permit: Six days of leave for the murderer of Juan María Jáuregui for "good behavior".

@Markoff said:

so after quarter of sentence can good behaving mass murderers in Spain enjoy time outside? insane

the movie shows him alone just visiting mother, not going outside only to see the victim

You must be an American. There's a huge difference between the Fox news "mass murderers" and the separatist Basques. For starters, the Basques are an ethnic group that was mostly self-ruling until the WWs. They were ultimately crushed, maltreated and murdered by Franco (along with other minorities). That their spirit was still burning when Spain became a democracy is a tribute to their proud heritage. The members of the ETA that were transferred to that particular prison in the movie were those that had already renounced the ETA's methods. In other words, they agreed with the methods of Maixabel and Juan Maria, who were also separatists, but not violent (think Mahatma Ghandi).