Is it paranoid to think every stranger you meet could be a serial killer?

its not unreasonable to consider the possibility that anything human might be, stranger or someone you think you know very very well, alike.
on the other paw, to assume anyone would have to be, for any reason other then conspicuously suspicious behavior, yes, that would be.

 

this reminds me of the wolf meem: "surrounded by enimies? good. that means i won't have to chase them down."