@sun
I can't respond to your question but this though came through my head.

>whether you call them "evil" or not, are irredeemably morally alien to humans
Be it evil, good or moral, all these three words are based on social concept that can and have been altered in history and still is today.

Meanwhile in the mango it's clearly stated that the way "demons" survive is by parasiting other lifeforms until extinction, which makes them defacto incompatible as humans evolved to the opposite of that.

My point is that there are physical, tangible aspects, in each types of lifeforms that can't be altered by moralistic judgement.