As an aside, I've been thinking about why it's acceptable, still, to slaughter zombies, vampires, etc. even ones which are sentient, and the answer is that, I think, there really haven't been many, perhaps not any ethnicities or the like characterised as literally predators on all of humanity - stuff like Blood Libel and accusations of cannibalism have absolutely bordered on that (indeed some vampire portrayals skid dangerously close to Blood Libel) - but by and large, no-one actually thinks "X people are literally gonna eat me!" or "Y people are literally going to drink my blood and turn me into one of them" (tbf I may be underestimating conspiracy theorists but I don't think that's ever got mainstream).
So I think it's still broadly acceptable to have for the slaughter:
1) Beings that basically want to eat humans (and human-like beings), either out of necessity or simply because they like it. Vampires, mind-flayers and their ilk. As soon as you put in something which means they don't have to, though, it gets more morally complex.
2) Beings that want to transform humans (and human-like beings) into more of them (zombies, the Borg, etc.), though depending on whether anything is retained this can get more complicated. But even on the Enterprise-D nobody was crying when the Borg Cube self-destructed, even if they were disappointed that there wasn't a peaceful or less fatal solution.
3) Beings without free will that are designed to kill or harm humans (typically some kinds of robot).
4) Beings who are inimical to human existence and impinging on it by choice. This is a weird thing that rarely comes up in D&D, but we're basically talking being who like, damage reality, or just cause humans to sicken and die by their presence, or want to terraform the planet or the like.