I don't believe that the video games totaly contribute the violence in school shooting. Maybe a good share of that.
Besides, I love violent RTS (real time strategy) games (as expected, explosions, human screams of death... otherwise the battles are unrealistic). But I never bully a single person, let alone on killing.
In the case of Cho, he has an antisocial behavior. So there. I wonder who pulled the trigger of Cho's rampage...