No matter how many violent, realistic computer games a kid plays at a young age, if he's been brought up properly then he knows the difference between reality and fantasy, and knows that going out onto the street and kicking grannies is wrong.
These kids who go out shooting people know exactly what they're doing. By all means, express doubt over the quality of their upbringing, but singling out computer games as a direct cause of their actions is just silly sensationalism.
I've played GTA quite a bit, and I'll say that it's a fun game. It's addictive, it's violent, and it's often pretty gory. The 'missions' on the game are inherently, and morally outrageous. But it's a game, and never once have I felt the need to go and kick a prostitute, rob a shop, or procure a load of hand grenades and start a violent escapade involving the police, FBI, and army. In the game, the consequences are (usually) "Game Over", and you start again, whatever. It's blatently obvious to even the most deluded juvenile delinquent that in real life, there are consequences of such actions.
A lot of the kids who are allowed to play 18 certificate games at the age of 5 are allowed to do so because their parents don't give a damn. The same parents are unlikely to give a damn about teaching their kids the difference between right and wrong, are unlikely to care as much about their childrens' education, and are more likely to live in a neighbourhood where crime is prevalent.