An abusive childhood experience.
A neurological defect
An expression of spiritual evil
Violence is a natural occurrance.
Yep, and to add to it from an anthrapological point of view, most cultures have a correct way for boys to express their violent tendancies, usually without killing each other (the next tribe over might be fair game, however). Our society is telling them (mostly, football notwithstanding) violence is unacceptable so you have no outlet, now go watch mass murderers, serial killers and gangstas get rich on tv and in your games. Sending mixed messages here? no wonder our kids are killing each other
You can have peace. Or you can have freedom. Don't ever count on having both at once.
Robert A. Heinlein