[NOTE: I am plan on doing more research on this topic for myself.]
I touched on this subject in an earlier blog of mine, but after thinking about it some more, I realize that this just can't be blamed on poor parenting; it can also be blamed on sociopathic behavior.
Because when you hear about someone stabbing another gamer over a video game like Counter-Strike, that's because they are probably sociopaths. Who else would do this? "Well what about short tempered folks?" Usually short-tempered folks just resort to punching when they get rage-faced, this is stabbing.
SCENARIO: Sociopath likes to watch violent flicks or play violent games and they're bullied at school. They're probably going to be influenced by what they see. Not the developers fault as this world can be very nasty and as artists, game developers like to reflect that. Freedom of expression.
The solution for this would be to remove the sociopath from the violent media. Why should one bad egg ruin it for everyone? "But what about bad parenting, Mouse?" Then take the child away from the parents/legal guardian. Not easy, nothing is. Hard work is the key.
My point is, we can't just say that gaming can't influence violent behavior, because it can influence those were were unstable to begin with. I'm not saying it's the only thing that can do this, even a tennis game between a sociopath and his friend can trigger homicidal tendencies.