I pretty much agree with zsouthboy and Borgleader.
There's always these rare cases of people who can't distinguish games, movies ect. from real life, but that doesn't mean that an otherwise great game, enjoyed by millions of people worldwide, should be banned.
I've been a pacifist for almost all of my life, but still I find it extremely satisfying to cut a guy in half with a chainsaw and teabag his corpse... in games!!!
A lot of studies have shown, that we humans have urges to kill and are attracted to violence (just watch a car crash on a large road or all of these gross websites with dead people). And that violent games and movies is a civilized way of "pleasing" these urges.
It's just like with the Columbine (and all the other high school massacres) where Golden Eye (that awesome N64 FPS game), Marilyn Manson (yeah, their music suck, but doesn't encourage anybody to murder) ect. was blamed for the killings.
This was even by educated politicians. You would think they were wiser than that.
Oh, btw, I haven't even had the opportunity to try GTA IV yet.