Sites depicting necrophilia are sick. No doubt about it. Did any of these sites make him commit murder? No.
John Lennon's assailant claimed his actions were a result of reading a book, The Catcher in the Rye. Some people were for the banning of that book, but the book didn't kill Lennon.
I don't believe there is a way to 'killer-proof' the world from these types of people. There were child molesters and murderers and necrophiliacs before the internet and I suppose that there may be a case made that there has been an increase since the internet made depictions of these readily available. But against what data? I was molested as a child, but back then you didn't talk about such things. Back then, in a lot of families (well, mine anyway), you weren't a victim... you got in trouble. It was not reported as often.
I think a killer will eventually prove himself, unless he seeks help.
Just my opinion.

__________________
Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these