I think you have to separate the question of "what influence (if any) do violent videogames have on the mentally ill?" versus "what influence do violent videogames have on normal people?"
To the latter question, we have already seen the largest study in human history over the past three decades -- a sharp increase in violent videogames (and TV/movies) and yet, as another poster said, if there was an influence, the world would look like Mad Max by now. I have done all sorts of terrible things in the videogames I've played, and yet, funny enough, I'm a completely nonviolent individual in real life, I've never even been in a fistfight. In the game Shadow of the Collossus I always thought it was fun to shoot arrows at my horse. The idea that I would ever want to do that in real life just because I thought it was funny in the game is preposterous.
There's a hilarious game called Surgeon Simulator that came out recently. You can do funny things like remove a patient's stomach and throw it at their head. That may not sound funny, but it's funny in the game. Would it be funny in real life? Of course not. Do people playing the game have the misperception that it would be funny to do that in real life? At least among the non-mentally ill, I'm quite certain the answer is no.
Now when it comes to the mentally ill, maybe there's something to that, maybe not. I don't think there's any evidence to support that claim yet though.
To me the real absurdity of the violent videogame debate is the fact that the people (especially politicians and media commentators) who keep claiming violent videogames cause these tragic events like Columbine or Newtown are the same people who blatantly ignore the much more obvious and clear causes and influences. Like how the media's saturation coverage of these tragedies inspires copycats. And the antidepresssant links, etc.
The present and future are full of fail.
