There is a new study out that says humans crave violence, like we crave sex.

http://news.yahoo.com/s/livescience/20080117/sc_livescience/humanscraveviolencejustlikesex

To quote the article:
Aggression is highly conserved in vertebrates in general and particularly in mammals," Kennedy told LiveScience. "Almost all mammals are aggressive in some way or another."
He added, "It serves a really useful evolutionary role probably, which is you defend territory; you defend your mate; if you're a female, you defend your offspring."
Even though it served a purpose for other animals, in modern human societies, Kennedy said, a propensity toward aggression is not beneficial and can be a problem
So, Gengis Khan good, Seung Hoi Cho, bad.

I'm sure this research was sponsored by the makers of HALO - it was always amazing to me why boys would want to play shoot-em-up games, but now I know. Its becos they cant get laid.