There may be other good reasons not to play violent games but I don't think they are responsible for the violence. Millions of people play such games without becoming violent and the studies that have been done have found no link between violent games and violent acts. Long before violent games there were violent movies (and of course there still are). I think perhaps these sorts of thing may be a contributing factor for people who already have psychological problems of some kind but for those who know the difference between fantasy and reality and are otherwise mentally healthy this isn't the case.

Though I don't play many games at all these days, there was a time when I was younger when I played many violent games. I would never dream of actually committing such violence though. I do think it is reasonable to follow the rating guidelines. Certainly young children shouldn't be playing games with graphic violence. However, I don't think even a complete ban would prevent something like what happened in New Zealand. His hatred didn't come from video games.