let me tell you a story...
frida night at work (i work at a sports centre) there was a young lad, probably about 10 who was doing judo. He came to talk to me and my mate, and my mate asked him if he could judo roll him. This young lad, after trying to judo roll him (and failing) proceeded to kick the crap outta my mate, genuinely being very violent, and even when we told him it was past a joke, he carried on kicking and punching him, it was quite disturbing, i actually had to drag him away.
games dont make kids violent, mainly because a "spectrum plasma laser extremo gun" is never easy to get hold off. However teaching kids martial arts (kicking and punching) from a very young age DOES make kids more violent.