The images on the screen aren't real. A rational person can differentiate between fantasy and reality. To say that a violent video game would influence someone's behavior any more than other media is completely illogical.
Books and movies have been showing scenes of intense violence (many of which are much worse than anything ever featured in a video game) for years. The argument that anti-game activists like to use is that the games make it "more real" by making it into an interactive activity that teaches behavior instead of simply showcasing it.
The flaws in that argument are numerous.
First of all, there's a significant difference between the behavior that a game teaches and the behavior that the activists like to link it to. The games encourage the player to learn a system of patterns that will allow them to master the game and succeed in its challenges. Non-players see these patterns in terms of the aesthetics and thematic content. Players see these patterns in terms of finger movements and reactions to motion on the screen. Playing Space Invaders won't teach you how to fly a space shuttle any more than Grand Theft Auto will teach you how to steal a car or win in a fist fight. Despite the vast graphical differences between the two aftorementioned games, the level of abstraction between the actions they represent and the actions carried out during interaction with them is identical.
That leaves the psychological effects of seeing the violent actions carried out in a realistic manner. Anti-game activists like to point out realistic blood and gore as a dangerous element in games, because it "desensitizes" the players to the sight of blood and gore, which, they claim, will make them less intimidated about practicing violent real-world behavior. This is stupid. Not only are other forms of media infinitely more realistic than even the most graphically advanced games, but the idea that the sight of blood or gore would be a significant detraction from committing violent acts is just silly. If desensitization to blood and gore was a deciding factor in whether someone is the type to go on a killing spree or not, every first-year medical student in the world would be a serial killer.
The other major point remaining is the morality question. Activists seem to believe that morality taught in games extends into a gamer's real-life world views by default. They seem to be underestimating not only the gamers themselves, but every other source of moral guidance that the gamers may be exposed to. With the possible exception of very small children (under age 10, let's say), it doesn't take a great deal of moral coaching for them to differentiate between the boundaries of a game and the real world. Children have no trouble differentiating between non-video games (such as Tag, Hide & Seek, House, etc) and reality. And those games take place in the physical world. If a child can understand the difference between a desk during class time and a desk while playing House at recess, how would they have trouble understanding the difference between a game that takes place entirely inside a a television set that they're controlling by pressing buttons, and the real world? And in adult terms, if someone can understand the difference in rules between playing Poker and Crazy Eights, how could they possibly have trouble understanding the difference between the weapons and people that exist inside the television screen while playing a game and the ones that exist in the real world?
And finally, pretending for a moment that games really can make people more violent and likely to mimic the behaviors they "learned" in the game... Why is it that Grand Theft Auto, a third-person game controlled with a joystick, has been used as a defense for shooting people will guns, while Time Crisis, a lightgun game that requires the player to actually pick up a toy gun and aim it like a real gun, has never even been mentioned?
this took forever to type, so if it happens to be really similar to something that was just said--yeah. too bad.