One possible answer is that it's a counter-adaptation to psychopathy. If some people are naturally monsters, being naturally afraid of monsters might be a good adaptation.

That's not to say we shouldn't treat him with compassion, or that we should treat him as a monster. But people who haven't seen the damage a predator can cause are often too glib; they literally can't imagine how dangerous another human can be.

You wouldn't pass any of the dozens of psychopath tests available if you lacked these traits, and you certainly wouldn't call yourself a psychopath.