As Matthew Lilard's character said in Scream:
'Films don't create serial killers, they just make them more creative'
I don't honestly believe that video games turn people into mindless killers, more like there are already some serious issues that these individuals have that just cannot be contained within the imaginary world of computer games.
One of the best films I saw about this was 'Elephant', where the director avoided making assumptions about his characters. There were no answers for their rampage, and in many ways I think that will be the case until the end of time. There will never be a definitive reason why people do the psychotic stuff they do, no trigger, no method to the madness.
Again, another more important point was raised, we probably do as much harm by eating a burger at McDonalds (if not more) as playing any old violent game.
