I have to say I believe this whole "playing violent video games makes you a serial killer" trend at the moment is total BS.... However, at 2 kids are like sponges, they learn to copy right and wrong.
My boys are 4 now, I let them play a little bit of Forza, Fifa and Mario/Kart and that is it - plus that is limited to a max of ~30-40mins a day if they have been good. I am a massive BF3/4 fan but wait till they go to bed to play.
As much as anything, rather than playing computer games I like to sit with them, do jigsaw puzzles, build lego, play hungry hippo, take them to the park and let them learn to enjoy being young.