The demons not being a pure evil race has been hinted at since a very long time ago. Especially after all that interaction with Felm. I’m not sure what you were expecting. We’ve already established that demons think and act like normal people, and there’s nothing explicitly preventing them from coexisting with humans. You think Usato will just go and murder all of them?Ah, yes, the reason "to survive". Welp, guys... That's it. It has been a long journey, but the author decided to provide a compromised future plot/ending where every race is happy. Most failed writing it, and only few managed to wrote it brilliantly.
Sigh...