The most violent I played was Mortal Kombat, later on in my pre-teens I got into GTA. As an adult I'd like to think I'm stable, most people consider me a nice, mature guy, and I'm far from being violent, I'm very much against war, the death penalty, and don't believe in resolving issues with any kind of violence or bloodshed. My brother was 10 when he got into Call of Duty, same with San Andreas (he prefers rural areas to cities), and I would say he's even more peaceful than I, very sensitive and caring, he actually defended (verbally) another kid from being picked on by his own friends, not falling prey to peer pressure, which is something I was proud of.
I suppose I attribute it to my mother, despite her downfalls, she was never a pushy mom, never told me I was wrong and she was right just because she's an adult, never hit me, always apologized when she was wrong, and tried to bring me up to be socially aware, she does the same and goes further with my other brother.
On the flip side, I have a buddy who happens to be incredibly violent and has a short temper, telling me he literally planned to kill his girlfriend's former boss for making her cry, he hates video games, and his dad was verbally and physically abusive to him and his mother growing up.