Could also have been cannibals, a lot of folklores talk about people who aren't really people that kill and eat people. Some versions of the tale of the wendigo feature whoever encounters them in their human forms noting that they knew they must be wendigo because they looked like normal people but something just felt wrong about how they behaved.

Uncanny valley could be at play in the ick you feel when you can tell for no apparent reason that someone's a psychopath or dangerous in some other way, the unconscious response to the things your brain noticed that you didn't.