You mention undead as acceptable "always evil" creatures. Why? If one is bitten by a zombie, he deserve to be cured, not shot. In the specific context of total loss of consciousness, one might argue that he is actually dead and that the behaviour is no longer sentient, attributable to the zombie disease, so it might warrant killing, but it would sidestep the issue since sentient-ness would have been removed. Let's take vampires: why would it be OK to kill vampire on sight? They are sentient and they have problematic dietary needs, but to kill them on sight just because they're vampire (who in 5e don't need to drink blood) ? And liches? Why kill them on sight? I don't think it's better than killing sentient orcs for being orcs. Once you accept a species as sentient, you can no longer justify killing them.