It's easy to kill innocent victims. Doesn't take a genius...just takes an evil piece of garbage. Just like the ones that prey on naive autograph buyers.
I understand that the rarity of it makes it desirable, but at this point and for decades to come, I would think that there's just too much of a stigma. It's not exactly something that you can proudly display without rubbing people the wrong way, and trying to sell it would likely cause even more of an outrage.
It's not a matter of rarity, and I doubt his writings are rare. I don't think anyone would proudly display it unless his package was stapled to it. But it's important to never forget evil people like this and their deeds—history forgotten is history repeated. Save it, put it away for a while, put it in a museum or exhibit, properly presented. This isn't just some killer like Charlie Manson.
I would think anything he signed is locked up tight in intelligence departments around the world. Probably not many if any exist that can be accessible.