There is the eternal debate about catharsis. There are two main schools of thought.
(1) Video games make you act more and more like your character. You practice violence so much that it starts to seem like the normal way to solve problems. In real life you may find yourself wanting to physically harm those who aren't doing everything just the way you like it.
(2) Video games provide catharsis. You can purge your soul. You can blow off steam. You can act out your darkest, most violent fantasies. Because you have acted them out this way, you won't ever actually do those things in real life and end up on Death Row.
I am not sure what to think. I suspect it is somewhere in the middle, and that video games make some people more evil in real life, just as they make some people more righteous.
May I offer a third option???
(3) Video games are a form or electronic entertainment, akin to movies, television, your ipod, ect. You don't see people shooting up schools because they watched a violent movie...(which, you have NO control over, and is oftentimes MORE realistic than your most visually intense RPG,) or because they heard a violent song or, even more rarely, because someone on tv told them to.
I'm more likely to do something I shouldn't if my game DOESN'T work properly, like lag or slowdowns or CTDs. But if it's running smooth, (which would make me happy to begin with), I'm usually calm as a tree while my character is murdering people in their sleep. And I usually don't go around murdering peoplein their sleep. In life, or in Oblivion.