I'm relatively new to videogames, having only seriously played them (at least the violent ones) for a couple of years now. And I really don't think I've gained any more violence in that time- I grew up in a family that did a lot of hunting, so I've always been familiar with guns and I'm not particularly phobic about them, but I'm just not a very violent person in real life, although I'm extremely violent in games.
If there's anything I can say about myself since I started playing violent videogames, I think my tolerance for violence has risen. I used to dislike blood and any talk about dismemberment, etc., because it grossed me out, but now I'm more OK with it. (at least in a fictional setting. After seeing real-life footage of blood literally pooling the ground from some conflict in... Serbia, I believe, I was just as horrified as everyone else, although perhaps not quite as grossed out.) However, this doesn't make me more accepting of violence, just gave me a stronger stomach.