i feel like paperwork comes with that kind of stuff. like the kind of paperwork you could show to clear up the nagging question of why and how you have human skulls around your place.
the skulls themselves aren't particularly suspicious or a crime in and of themselves, but it does raise questions. the kind of questions you probably want paperwork for.
And to be fair, the odds of an investigator or forensic pathologist asking to see the darn thing are greater than zero. In fact, you could be saving someone the trouble of exhuming a whole coffin, just by having the thing lying around.
I'm actually a fan of Cannibal Corpse, I saw the songs I didn't recognize and got excited. Turns out there was a new album out though, so it's not all bad.
So yeah, Collecting guns is a thing people of means do in the US for fun when they're not keeping them around for hunting and dissuading vermin. And skulls can be obtained for a variety of reasons. (I had a set designer with six-plus skulls he'd use as molds for dozens of plaster skulls when he needed them for film settings.)
Unless he's committed an actual crime, or even just needed a license for collecting guns in his county when he had none, this is essentially reporting a celebrity likes the wrong kind of porn.