One of the most disturbing things for me was (Spoilers)
Obviously the fire extinguisher scene, the realization that he killed the wrong man but probably not an innocent man, the rape scene was horrific but the thing that really got to me was when you see another man enter the tunnel, sees the rape occurring and simply turns around and leaves.
That was a devastating critique on society as a whole and that scared me on a very real level, sorry to get political here but everyone has been so, for the lack of a better word, neutered and conditioned by the government to believe that only elected officials ie cops, can enforce the law that most people are completely apathetic or ambivalent about anything, their train of thought, it's not my problem, that's what the cops are for, I'll just keep my head down and I'll be right.
In other words, if the cops aren't around, no one is going to do anything and I feel like this kind of apathy is what allows a lot of criminals to thrive in the first place, society as a whole, has lost their collective backbone, most people have the attitude of just keep your head down and no one will bother you but that's the problem in the first place.
People have too much faith in the government to protect them and not enough courage to protect themselves and others.
I don't suffer from insanity; I enjoy every minute of it.