Seeing as it’s a question about sin, you’re best asking a theologian rather than a board full philosophers. Perhaps God disapproves of this “virtual murder”, perhaps not, though I don’t remember seeing him mentioning anything about GTA in the bible. 
However, it’s a question of morality - rather than sin (two very different things in my opinion) - then no, I don’t see anything wrong with running over pedestrians that don’t actually exist beyond the code that gave rise to them. You could even argue that you’re acting morally, because everytime you turn on the computer you’re giving them a life - albeit a short and bloody one - that they would never have otherwise had. You create Vice City when you like, breathe life into its inhabitants, then murder them according to your will - much like God himself then? No, I fear I’m taking the analogy too far.
Anyway, as I see it, there can be no thought crimes. I can imagine murdering someone, or even act out a murder on a computer, but so long as I have the ability to discriminate between the world that exists on my side of the computer, and the world that exists within it, then I fail to see a problem. Morality has more to do with the treatment of other people than with the treatment of ones self - so long as your actions (or thoughts) do not adversely affect another individual, I fail to see why you should have a case to answer to.
Sorry, don’t quite get you.
Thinking about murdering someone (or acting it out on a computer screen) may cause you to murder that person at a later time, but when physically going through with murdering someone, obviously the fatality rate is going to be much higher.
There’s been much made of the connection between video-games and real-life immorality, but, as I see it, it’s a pretty thin one. The advocates of this theory fail to differentiate between cause and correlation: I would argue that a naturally violent person is more likely to play violent video-games, but I don’t think that playing video-games are quite so likely to turn an otherwise passive individual into a mass-murderer.
Living in a society with lax gun laws, for instance, is far more likely to see a congenitally violent person (whatever that may mean) act upon his murderous desires than all the video games in the world combined. People may not kill people, but people cannot kill 20 people a minute with their bare hands.
Er, sorry for going off topic… 
