Look into the research of Lt. Col. Dave Grossman. He has written several books on this very topic. I've attended a couple of his classes pertaining to the psychology of killing and its effect on a person when I was still doing SWAT.

He cites statistics that show that during WWII, a very small percentage of combat infantrymen, actually actively participated in combat. Many simply hid, others would participate by firing their weapons but they did not aim at anything, while a very small percentage actually conducted aimed fire at the enemy. The Army looked into this and changed the way they did their firearms training, by switching from shooting at bullseye targets to human silhouette targets. This increased the percentage of soldiers who put aimed fire onto the enemy. The numbers still weren't high enough to suit the Army, so they switched to 3-D targets and this worked even better.

They determined that the more they "conditioned" the soldiers to shoot at something that looked like a real human being, the easier it was for them to actually do it in combat. Grossman says that the violence that kids are exposed to in the media, whether it is movies, TV, or video games, is actually conditioning them to be more violent.

As far as the gang members that Thump mentioned, they don't need the video games, they see the real thing on a regular basis from a very young age.