I think of it in two ways :
1) Video games don't make a person a bad person. They don't make you violent, or want to go on a mass killing spree. Either you are that way inclined, or you aren't. It is not the gun that kills someone, it is the person who pulls the trigger. I have played GTA SA a lot. I like it. Yet I have zero desire to go out and run down old grannies. I know the difference between a game and real life. It's fun to do in a game, because no-one in real life gets hurt.
which leads me on to ...
2) It is a form of escapism. GTA series allows you to play out being a gangster, going on a killing spree, stealing cars and everything else you can do. It allows you to engage in role play. In real life, you might be a nerd who could barely rip open a packet of crisps, but in a computer game, you are a badass dude, and no-one messes with you! Sometimes being able to role play like that, is good for you. As long as you understand it is a game, and not something that is acceptable to do for real
How many parents give their sons toy guns to play with? And of those boys with toy guns, I bet all of them run around with their friends going "bang bang - you're dead". Yet, would any of those parents want their little boys to actually get a gun in real life and go around shooting people? I doubt it.
And let's look at Elite and all the other space games we like to play. Being a pirate is an option. In the game, you deliberately attack others with the intention of either killing and stealing their cargo, or just stealing their cargo. That is OK because it's a space sim ... right? How is it any different to the GTA series you are so shocked and horrified by? How is it different to giving your son a toy gun and letting him run around going "bang bang, you're dead"?
GTA series IS A GAME ... IT'S NOT REAL. As long as you treat it as a game, I think it's OK.