You've killed me in CSS before nina

And I think they do. Obviously it depends on the mental state of the person, but to deny that violent video games and TV shows etc. have no impact on the likelihood of committing a crime is a stretch. If you can differentiate between reality and fantasy then you'll be (presumably) able to suppress the ideas of theft/murder etc. as morally wrong but playing violent video games probably increases the frequency with which you have to do that.
I checked the literature on this, since it's a fairly controversial area and a few studies have found statistically significant correlations between electronic media usage and violence/criminal activity. That's not to say it's causative (and we'll never know for sure) but I suspect it does have an effect.
On the norway shooting; that guy was already mentally disturbed and COD might have been the tipping point but you can't point to it as the defining factor.