Hmm, I can easily imagine that "messed-up cannibalistic half-sentient beasts" could translate into "inauspicious" and "unnatural". Maybe that's what they meant by inauspicious to start off with ("They'll eat anything, even each other! They have no virtue at all!") and its meaning kind of spread into being generalised bad luck and ickiness. I feel like even the violent urges could tie into this, actually, in a misplaced kind of way: every time Shen Wei has a strong feeling, he links it to the hunger that drove him to kill and eat other (dirty dirty) gui, so his reaction is to repress repress repress and hate himself for it. You know?

(Does Shen Wei eat food in contemporary times? I can't remember.)