PARENTS ARE.

It's the way that they raise their kids that can cause them to be violent.

But since some of you guys still think that the reason is video games:

OoOoh look at me. I'm playing Kung-Fu Heroes. Watch me punch each and every person on the screen so I can proceed to the next level. Oh, you know what? I'm gonna go to school tomorrow and sucker punch a kid in the throat because of my video game! I'll be just like Jacky and Lee because my parents never taught me any morals! :)