Even if some spree shooters used video games as a "training aid" -- and it is clear from their own testimony that some have -- even if a vanishingly few unbalanced individuals are induced to real-world actions in imitation of video-game violence (and I have seen no evidence that even the spree shooters who "trained" with video games were turned to violence by them), the net effect of virtual violence is to decrease the amount of real violence in society and the occasional exception is akin to a side-effect of a beneficial prescription drug.
(Truth in advertising: I'm an old-school gamer. I never got into first-person shooter or other computer or video games dependent on reflexes, but they weren't around when I was in you youth. Hex-grid wargaming, D&D, the odd turn-based computer strategy game, M:TG (before they ruined it in Mirrodin block), . . . )