They actually did an experiment whose aim was to prove that violent video games make someone more violent. It involves someone playing a violent video game, and then taking part in a controlled quiz. They ask the questions; if the person gets it right they give them jelly beans, if they get it wrong then they have to dip the persons hand in ice-cold water for however long they wish.
It turned out that those who played the shooting video game for the longest were the ones to keep the hand in the icy water for the longest, i.e. displaying violent/sadistic qualities.