All I am going to say about this, is games have an age rating, it is there for a reason.
This also prevents "kids" from buying said violent video games. That's where parents come in...
You know, I was at my local game shop just the other day.
There were 2 young boys, that looked to be no older than 8 or 9, after buying the new Black Ops and Halo 4, this, is the problem.
When I was that age I was playing Pokemon, Crash Bandicoot, Spyro and SSX, why? I actually had a parent that cared, and who wouldn't allow FPSs.
It is NOT the game its self.
Also, I do not see how twiddling your thumbs with a controller will train you to become a serial killer, as it would most likely decrease your physical strength, and you probably can't get a hold of the weapons if you're a kid playing video games.
Also the guys motivation was NOT video games, as has been mentioned several times in earlier posts.