I came across a condition referred to as Wendigo psychosis it's when someone who resorts to cannibalism to survive begins to believe they are a Wendigo. This usually results out being driven mad by guilt.

A wendigo is a mythological creature that starts out as lost person driven mad by hunger and turns to canabalism to survive eventually becoming this less than human creature.

Though lately I've been wondering what if through out history such disorders are the source of our myths? What if serial killers and pedophiles are the ones who are at source of our boogiman legends?

I think it's likely that schizophrenics were mistaken for being possessed or what have you I'm not trying to be offensive but given how uninformed previous cultures were likely following the logic of "If it looks like a duck" I can imagine this being possible.