I don't know why it has to come down to right vs left. The whole issue of violence, and gun violence particularly, ends up being framed to support a pet issue on someone's agenda. Blame the guns, blame the video games, blame the movies, blame the religious zealotry but by all means never mention that humans are by nature cruel and violent and not everyone is able to retain the ability to suppress this.
Showing the pictures will only fan the emotions and end any possibility of an intelligent debate. Maybe that's what Moore wants. Can't sell documentaries about subjects that don't keep us on edge.