I don't play extremely violent games like GTA and the likes, but sometimes I play games that can get violent. When you play a really good, well thought our game that you can just immerse yourself into, all your worries and problems just go away since you are focusing on being that character in the game. When you are really into the game, you do start to feel what you would feel if you were really in that situation. Like, killing a bunch of zombies, you might feel your adrenaline rise or even feel fear. Thus, the fear drives you to kill more zombies. If you are under a lot of stress, pressure, or angry, this adrenaline and fear will take over more and your feelings will change. When the game is over, you begin to wonder why you felt that way before you played the game. It really is theroputic to kill zombies and such. You can just feel your anger drain away as you put all your energy into being that character on screen.
However, games really do not provide the solution to the cause of your original stress or anger to begin with. After a gaming session though, a person could be calmed down enough in order to think more clearly and deal with the cause without doing something stupid. So, gaming could prevent doing stupid things in life.
Good question. I know that violent games were the blame of Columbine, but I think that's horse rubbish*.