My view:
People kill people in games because they know they're not real, and it's fun because it's like you're living out an action movie when all the police chase you. It's nothing more than a more mature form of the imaginary games you play as children. They also do it, not to rebel, but to play around and experiment, and see what happens.
The violence doesn't desensitize you- because instincts and feelings can't be numbed down- well maybe the latter, but never the former. When a huge catastrophe that affects millions happens around you, you're not prepared- you're scared. You panic, worry, etc. Basically in games you do what you want because it's guilt-free, there's no consequences, it's not real, and it's pretty much Groundhog Day because whatever happens, you just start all over again. The "people" basically dummies or models used for you to practise on. Not for real life- but for in the game- because moving your thumbs around a small area isn't going to make you be able to beat a man to death, or stab him, or get your hands on a variety of heavy weaponry. Everyone knows games do not equate to real life.
Like I said before- it's just Groundhog Day the Video Game. That's it. No fancy psychology or anything. Just a 15 year-old schoolboy who plays games, and sat down, and took a few minutes to think about playing GTA, and to use common sense. There you have it. I can't believe taxpayer's money is wasted on this !@#$%.