I play CoD4/5 all the time, and I have Halo 3 as well.
Guess what? I'm not violent at all. (Sometimes I beat my younger annoying brother up, but I've been doing that since before I owned video games, much less violent ones, and I'd still never kill him.)
I think people just always want to find something to blame people's actions on. Right now people want to blame video games.