User blog comment:SlenderVSJeff/Slenderman or Jeff The Killer?/@comment-3285977-20120828175515/@comment-4295646-20120828182004

Very true. But that's not too substantial when the objects of belief are known for pretty much nothing but kidnapping and murder. So, until they actually kidnap or murder someone, it's difficult to say that they really exist anywhere.

Anselm talked about your point, in his essay concerning the existence of God, but later criticisms brought up the distinction between "X exists" and "the idea of X exists". With the two dudes in question I think we can only admit the latter.