The whole "videogames cause violence" myth is such a cliché that it feels almost unnecessary to debunk it. The Escapist could probably fill an entire article in itself with the numerous variations on this myth: from disbarred attorney Jack Thompson's ludicrous campaign against Grand Theft Auto, to the story of the 2004 murderer Warren LeBlanc and his supposed obsession with 'manhunt' (which, by the way, he didn't even own a copy of the game).
The blunt fact is that a kid who takes a cue from Commander Shepard to shoot up his local Wal-Mart wasn't really functioning on a "normal" level anyway. Correlations between videogame violence and its real-world equivalent do exist, but only when there were underlying issues in the first place - tendencies that conceivably could have been triggered by anything, and which really can't be blamed purely on Microsoft or Sony. This proves that not all the violence comes from video games.