I suppose it's possible they actually exist, same as any otherkin. I haven't actually formed a concrete opinion on that. And besides it'd be just an opinion.
As for what they'd look like ... The scariest and most likely answer, is "just like the rest of us". And I don't say that just to be scary. I just mean they could look like anyone. I firmly believe that the level of evil in a person does not determine how they look.
In any case, demon or not, there are some seriously sick, twisted and evil people out there. I know, I've met one or two of them (in my former professional capacity, as a cab driver).
I hope this response helps.