First. Please do NOT put spoilers in a post without warning people. Good christ, how often should we say this?

Second.

SPOILERS FOLLOW ----


Are you sure he killed that girl?

I think if you watch it again you'll find that it wasn't him.

Third, the point of the film is not that he is without responsibility or blame. The point of the film is that there are no easy answers and that there are ills throughout society that are in need of correction.