I know it's an unpopular opinion in a game's forum, but I do believe video games can make kids violent and desensitized. Of course it doesn't apply to all the cases, but I'm sure it happens a lot. Specially to those who practically live in them. I'm not saying that means they're making killers though.
I feel like this again is a generalization that is not helpful. Let us put it that way instead: IF the kid has problems and has no way of resolving the problems, of course it will be reflected in the gaming habits as well. But it will not be the cause if ot, imho.
One more thing: I personally handle all video games as a pure result of Will. It is a playground for the mind, wholly and utterly. Bullies and stronger students, outside activities and sports might dominate this so-called real life, but for those that did not find that so compelling, there was little interactivity before video games (books, but that is a different - not better, not worse - experience). Now this new field of experience was born, and millions of kids want it in their lifes. But that does not mean that they cant encounter the same problems, for example on Xbox Live. If your escape plan turns into a verbal abuse by angry 13 year olds over a Black Ops 2 match, then the situation will be just as fucked up as it was if you were on a playground, getting bullied by others.
The key is still communication with the parent. No substitutes for that.