Let me re-count some personal experience that may seem irrelevant, but it kinda makes sense in my head
Back when I was like 8 years old i'd see kids get into fights with each other.
They'd grab each other in an arm lock and just pull each other and that is as far as it would go. As I grew older, these kids started to be influenced by Hollywood and other such mediums. These simple scuffles started to turn into serious stuff. People would get punched in the face and kicked in the balls. One kid once used my cricket bat to hit a guy in the head, cos said guy called his mother a tennis ball. (lol)
I think media has a lot of influence on kids and computer games may be no exception. It certainly de-sensitizes kids in my opinion, since i'm also very de-sensitized by it as are people around me.
Some people are still effected by blood and gore pretty badly in movies but those same people aren't effected at all by anything similar in games. Like my friend doesn't want to watch Samurai X with me cos he's squeamish about the blood but he then goes and plays Dawn of War which has some pretty gruesome killing scenes. He has been playing games a lot so its understandable that in his head he may just accept the bloodshed in a game.
I dunno how it works psychologically but i'm sure games play their part in influencing kids as does other media.
Whether they act upon this and bring out this violence is totally down to their nature.