Evilness of the human nature

I watched The Girl with the Dragon Tattoo tonight. It made me to contemplate on the evilness of the human nature.

I remember reading a police report about guys who would threaten prostitutes after using their "services". Some tried to kill them, cut them, rob them......

Why are they like that? Is this the innate part of the human nature? Why are they doing these? Because they can get away with it? Do people get more pleasure out of doing something that's prohibited? Even if it causes pain in others?

I'm not afraid of dying, but I'm definitely scared of evilness of the human nature...
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Last night as I was tossing and turning on my bed due to the nightmare-ish movie, I realized something else about the human nature. When I was watching the movie, I was getting so angry at a couple of evil characters, I imagined horribly hurting them. Then I thought, "Wait, is that justifiable?" Wanting to hurt them because they hurt others,...is that okay?? I'm sure the general public would join my side to hurt those who hurt the innocent victims..but is that really a right choice?
Also how did those characters (pretty realistic) arrived to the point of such hatred and anger when none of their victims caused any pain to them? Can we train our brain to be so angry toward someone whom we just met?