Becoming desensitized to violence could make one not be affected by the daily dose of violence on the news, in life, etc. It doesn't mean that someone would go out and mass murder people.
If you accept the premise put forward in the OP then you must also accept that it was Sarah Palin's fault that Gabby Giffords, et al, were shot. After all, putting targets on politicians and telling people to lock and load or whatever idiocy she spouted is just as swaying to the mind as killing cartoon characters.
I agree, in that I don't think that even excessively violent video games are what triggers these killing sprees. Any ideas?