I know from personal experience that porn can create and fuel violent tendecies in users.
The FBI knows as well. Researchers know as well. Ted Bundy experienced it himself, and his numerous victims know the truth all too well.
It's a fact, but one society wants to ignore in it's hedonistic heyday.
It must be some totally different kind of porn than I've seen. The fact that someone would look at violent porn indicates to me that he is violent. It seems to me that it is false logic to find a commonality like porn in prisoners and they conclude that the porn is the cause.
It's like finding that people in bar fights are very likely to be into weight lifting, therefore weight lifting causes bar fights. The numbers imply a proof but it is a faulty proof.