Blaming popular culture for the actions of a phycopath is just a way to pacify the masses. If you have something external (like movies) to blame, you feel like you have more control over the situation (ban the movies!), and thus the world is a little less scarry. Case in point: Columbine. Do you remember that the media reported that the killers were "loners"? Years after the fact, students from Columbine had something completely different to say. The killers from Columbine weren't loners at all, they had lots of friends. We were told that these kids were loners so that we would have the false sense of security in thinking that only those "wierd" kids could do something like that.
In my parent's neighborhood a few years back, a mentally ill teenaged boy got a hold of a gun and shot the town sherrif in the face. It was not highly publicized (relatively speaking), and what do you know, no one blamed popular culture. If it had made national news, the good bet is that people would be talking about the possible connection to video games, TV, music, yada, yada, yada.