I actually can't believe people can misunderstand incredibly clear and straightforward storytelling this extremely.

There is not a small number of demon defenders that have seen the show - they definitely didn't read the manga, as it is even more clear what the demons are since it isn't dubbed over and you don't have plausible deniability of misunderstanding the tone or words somehow.

This is like thinking that lava is redeemable if you are just friendly to it. Demons don't care about who you are, what you are, they view humans as less than insects and have absolutely no framework for anything that could be called morality.

There isn't even a single moment that has you "question the morality of the demons", the closest it gets is showing you that demons can fake the guise of humanity with the entire purpose of killing/eating/destroying all the humans around it.