Surely the responsiblity is on the parents not to buy $50 violent games for their kids. They say kids can buy the games online with no age check, where does a 15 year old child get a credit card from? His parents.
I think childs do need to develop their own critical thinking: choosing what they can or can not play without their interaction if useless, if not dangerous. If I found my child playing, let's say, a Silent Hill game, I would simply approach him and start talking about about the aspects which strike me, and what I find disturbing or even interesting. If he values your judgement he will tend to like what you approve, and he'll probably avoid playing it if you feel clearly unpleasant. And if you think that it's really more then enough, tell him directly, explain the reasons and put the game away (a Silent Hill game is reasonably easy to explain, if only because of possible nightmares ;) )... but that should really be an exception and not the norm. It's still an artistic creation after all, and a parent should never react as if they discovered, let's say, that he has a habit of beating fellow people. Otherwise, if you let him leave in a world of Pixar and Disney, one day they will still discover trends you find out of line without having them known what you think about it (as his model, not his autority), and they could like aspects which were never discussed before without him knowing your opinion on the subject.
GTA is almost like an action flick or a gangsta movie, something that any child saw at some point (like in the famous 'Home Alone' scene). Personally, as a child I found most of them quite boring to watch, and the GTA series is actually interesting not much because of the storyline (even though in San Andreas it became quite good) but because of the gameplay. I would regard it as almost okay to be played with a parent, and almost interesting in a way since you have free will and could try to avoid 'unnecessary' violence while doing the missions (such being careful in car chases and shootings), and as long as you avoid some of the most deprecable actions. However, missions tend to became quite immoral even for TV standards if you relate the main character to a movie star and not the villain, and that's the off putting aspect which makes the game adult only without a reserve.
those that do get violent are probably that way becuase they're parents obviously pay no attnetion to them so they can just sit playing what ever the hell they want and get ideas from it, not motivation.
I agree, there are latent tendencies and reasons if you are going to commit such crimes.
However, I don't consider most videogames to be written good enough to get ideas from them. ;)
It has often been true for novels though.