To play violent games, one is required to desensitize. If I think about killing insects in my home too much, I go "Oh. Shit. I just finished that fly's life.
". Whereas in video games, I'm an evil little bastard. Yes, I'm evil on fable; Yes, I killed the first NPC on Doom 3, and I enjoyed it. This hasn't effected my life at all, as I play video games purely for fun.
It all depends on the person. If the person in question wasn't particularly mentally stable before playing 'Violent Killing Spree 4: Dub Edition', perhaps they have a slightly bigger chance of being affected by it.