L'instinct de vengeance

The story is really simple in this jail topic, at least the first two thirds. Simple story where the viewer has many difficulties to get where it leads to. What's the purpose of all this? Only by the end, you may guess, ONLY guess, without being totally sure...It should have been explained a bit better. But it is dark, gritty, without any morals, not a Hollywood style ending too. Good directing, acting, but the time and efforts the audience try to undertsand is not spent to entirely appreciate the whereabouts of this sophisticated simple story. Even French film industry don't present stories this way. But it is good, don't misundrestand me.