I read a report on a study among 130,000 people that concluded that violent video games increase the frequency of aggressive thoughts, and decrease pro-social behavior in everyone, regardless of age, sex, or culture. I'm inclined to believe it's true, even without seeing the data. Psychology is a "soft science," but it is a science and not an art.
The reason why I believe it's true is because I believe that we always absorb what we're around. Even if we try to avoid it, and even if we can completely transcend a situation, all you really are doing is slow the rate of absorption.
That being said, I don't believe violent video games are going away. They are entertaining. They are a great way to release frustration--even if not the most socially-healthy release. In many cases, violent video games are purely used as a social catalyst--like a movie, but usually simulate conversation.
Sadly, some people who play these games argue that it doesn't affect them. In reality, they are lying to themselves. You can't be given information, in any form, and not be influenced by it. The effects might be small--even unnoticable--but it is ignorance to say they aren't there.
I know that violent video games affect me, I know how much they affect me. I think the important thing when playing violent video games is that you counter those negative effects with positive ones. So long as you have positive social outlets outside of video games, you can have positive influences negate the negative influences that video games some times have.
It's like TV, or going to the movies, or just about any popular form of entertainment. There will be bad influences on you. Just make sure you have good influences outweighing the bad. Go outside, run, play a sport, go to a party, read a book, and doing things to keep your character and interests well rounded will negate the negative effects of violent video games.
One last point I'd like to make: watch yourself. Put yourself in the same real-life situation as the character. If you were in the middle of the battlefield being shot at, would you really care about how many times you shot enemy soldiers in the head? Make sure you still feel remorse when playing a game. Make sure you recognize, consciously, that if this were real you'd handle it in a human manner. If you can't do that while playing a game, and you're getting so into it that you lose sight of reality, then that game is bad and you shouldn't play it anymore.
So, I don't think I'm going to stop playing Crysis, or Team Fortress 2, or Halo, or any game like that, but I do recognize that I could stand more positive influences in my life, since I have noticed a decline in my own "pro-social" behavior.