Not just humans - I think it's not unusual to see a sick animal, notice that it's "moving wrong", and feel a revulsion that motivates staying away from it. It's a very handy instinct if, for example, that animal might have rabies...

Edit: I agree with you, I'm just expanding on what you said.

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.

IIRC Sapiens, Neanderthals and Denisovans coexisted for some time, so it could be not about things that aren't human, but humans that are different. To this day, xenophobia and ethnocentrism are common attitudes.

I just saw this meme, though it was funny. Showed it to my friend and he said almost exactly this.

It makes perfect sense now that I hear it.

- things lizardman shapeshifters say

Why can't you believe it