I'm sure that stuff can be an incentive for some crack nuts, as a matter of fact I seem to remember a juvenile going a shotgun spree under the influence of GTA, but I'm not sure ...
Dont you think this is because it is easier to blame it on something else like a game instead of blaming yourself? Violence and killings have NOT gone up since such games arrived. The stats in alot of places actually shows a decrease which includes USA.
A mentally healthy person will in no ways take damage from playing a violent game as they can differ between fiction and reality. But what do you say about movies? There was the same hysteria when violent movies came out, but that as well turned to be false.
I can handle age limits though even though i do not believe that a underage will take much damage either.