I mean… Gnolls are evil. Basically all fiends are evil. Having evil races where killing them is almost always morally correct is not a problem in itself. The problem is making it so that some races are inherently evil without actually explaining why. All my examples have some kind of cosmic evil embedded in their nature. But that’s not gonna be the norm. If all your evil races and all your good races are so by nature without any way of changing that, it’s bad writing and bears the risk of implying that people are created either good or evil.

And as far as I can remember, that kind of explanation never existed for Orcs or Kobolds. They were evil by nature without explanation.