I believe the overly violent games such as, God of War, Gears of War, killzone series, ALL call of duty games and the like should not exist, its only purpose is raw violence with no positive meaning behind them what-so-ever.
atleast in games like mario, little big planet, or hell even wii-sports, you benefit from it.
Or for greater example... JRPG's, most JRPG's have a waterfall of emotions put into it, and it makes the players give a damn, think of them as interactive books, the JRPG genre and hell to some extent even MMORPG's and WRPG's have really great stories to them.
In conclusion, a peaceful mind makes a peaceful man.