I don't think it's the games themselves that cause the brain to be influenced with more violent urges, but rather the fact it's okay. I've never seen too much of violence, nor heard, of it rooting from a game, but I do acknowledge it happens and is possible. The fact in these games that we are able to kill and hurt with no consiquences, even rewards sometimes, is going to subconciously affect a person after a while of playing. It's been known that younger children are influenced heavily by what they're surrounded with, so it's very possible.
I have a nine year old brother, and I study his behavior when he plays a game. He gets irritated when he can't win, and even goes to the point of screaming at the television screen. It's hard to get him to even turn it off. Personally, I believe that a lot larger and worse influence is rather from television itself, as that seems to effect the viewers in more ways (Language, activities, ect.), but I'm not clear on just how concious we are of the violence we play. Military, criminal, supernatural, and just your regular guy off the street, these are images that we can connect to our real lives (Supernatural: Not for the elders, but for a child who still believes in Santa or Fairies it's more plausible). And as people know, what we connect to we better understand and connect with.
That means we're accepting these games as they are, even making them a part of daily lives in some times. Psychology in response to video games is a topic that I've pondered for quite some time myself, and I don't doubt many of my peers and elders have as well. Our subconcious could very well let this aggression sink into our minds, and leave us with very violent urges.
There is a flip side to this. We've all heard the stories of a child who, with pent up stress, finally bursts and kills both oneself, usually, and others. Violent games are an outlet for that, in the positive sense. They give us the opportunity to do what we cannot in real life, knowing that it's wrong. So how can we kill so effortlessly inside of these virtual realms? It's because we know for a fact they won't feel a thing, we won't be punished, it's merely a way to get out what we want to. I've done it myself, and seen it done. Some people, I don't doubt, even picture those they wish to harm as the doomed victim. But as for real life aggression, I feel like in this light, it won't add on. Looking at this positive side, it feels just the same as a child drawing an angry picture to let out their feelings. Only this is more realistic.
The fact they're growing more and more realistic every day with the development from new game companies is a bit scary. The dosage with these games, as most other things, should be contained and mantained at a safe level. While I'm not denying what I said before about it being able to do good, I'm also afraid that having too much is what causes the aggression. We get so comfortable with it, that we completely lose sight of reality and fiction, which has a very thin line as of late. It's a matter of how you deal with the situation I think.