I agree completely with Bella.
I am sure that everyone, at least subconsciously has had the urge to kill someone. A sane person would not dwell on those thoughts for long, sometimes not even letting them come in contact with the conscious mind, but a mentally ill person can't reason on those urges simply because they'd see nothing to reason about.
I strongly believe that if society had a little more compassion for paedophiles and such, there wouldn't be as many cases of child abuse. I've been talking to these people for quite a while now, there are some sickos (very sick sickos) but most are just trying to live their live the best they can. Self-loathing can be a very dangerous tool in the hands of a madman. The self-loathing ones, who have nothing to lose, are the ones that rape and kill.
Of course, I am generalizing here. There are many factors that can contribute to a person developing these desires. In a perfect world, the situations that create rapists, child-molesters and the like wouldn't exist; and a person would have enough trust in their family and friends to talk about their problems from an early age. But we don't live in a perfect world, and most of these people grow up hating themselves, and in turn, the world that rejects them. That, in my opinion, is why many of these people don't really notice that their actions are wrong; they simply don't value humanity.