My take on this has always been if a person is so easily influenced by media that they become a killer then that person was already batshit insane and thus something would of set said person off. Videogames, tv shows, movies, etc. They'd need to avoid most forms of entertainment. Crazy people like videogames as much as non-crazy people, it's not the games fault..it can't pick and choose what kind of person plays it. Hell I once shot a damned baby in the face in a videogame, but it didn't make me want to go out and kill babies.
I have no doubt the media can influence people, but if it's having *that* much influence on you then you have issues at hand that have nothing to do with videogames.