Everything you think about Game Mastering is probably wrong: Part XI - Orcs, the Undead, and Claims of Bigotry
Everyone has heard the conversation about orcs and goblins always being evil being a bigoted trope, but what about undead? " But undead are inherently evil! They are born of black magic!" one exclaims. Exactly why? The real answer is because there are a lot of Christian assumptions built into the game by virtue of the culture/era it was created in. The dead stay dead, as for all things there is a season. Therefore, they are evil because the gods will it, much like they will all orcs to be evil for their race.
Of course, other cultures have very different views on the dead. Some of them keep the dead in their home. But Aha! You might say those are the dead not the undead! which is a false distinction, up there with saying but Orcs are monsters not people!
But they are dead! It can’t be a problem to kill them again! you might counter. If science does manage to revive someone from cryonics there indeed will be people saying they are unholy abominations and need to be destroyed. We would call those people fanatics and bigots.
But necromancers are evil! which is true, because you make them so. It is called black magic, but why exactly? Raising someone from the dead is basically advanced medicine. True they will lose a lot of biological norms, but so? People who have suffered all sorts of real trauma lose biological functions. How many functions does a person need to lose before they aren’t legally "alive" anymore?
But the undead are monstrous! they are if you make them act that way. Orcs are evil too if you make them always act as evil, but why do you make them always evil?
If your party should find a sheltered valley and ask a peasant to shelter them from a storm only to discover the peasant has the undead body of their grandmother seated at the table helping to look after the children, would the party set fire to the whole hut? Even if the grandmother has to go to the village healer every day for a dose of blood to stay alive? If that is too far, how is that different from a sick person needing a recurring blood transfusion? If the grandmother would go insane without the blood, is that different from any number of health conditions requiring routine medication?
The correct, short, and pithy answer is that it is a decision for you and your group. The larger answer that is the actual point of this post is that the more you choose to humanize a monster the less of a monster and more understandable it becomes.
While I like “Aliens” as a movie more than “Alien”, Alien has a far better monster. The xenomorph in Alien is utterly alien. It isn’t an animal, it doesn’t eat people, it “disappears” them and leaves no trace. It is inscrutable (especially after they wisely cut the scene that gave it some kind of reason for taking people). In “Aliens” they act in a more understandable biological parallel. When they capture people it is to use them as incubators, like a nest of wasps. They have reasoning and goals, they not only have a queen who produces eggs (like a hive insect) but who cares about its young. It could attempt to negotiate with Ripley to attempt to save its eggs. The xenomorph is now far less alien and far more understandable. It's a dangerous life form that is hostile to humanity, but it's understandable.
It's possible to humanize space insects even more, Starship Troopers does this. By the end of the movie you realize the Arachnids just want to be left alone and the humans are fascist imperialists. The Arachnids live on a series of desert planets rich in resources that human colonists keep trying to settle. They eventually decide to send an asteroid to crash into an economic hub of Earth in what they call a retaliation and what the Earth government calls a terrorist attack and uses as a justification for a massive military action. Already your brain is drawing parallels to real world human cultures which makes the whole thing a much less simple setting to parse. In “Alien” there is no underlying baggage to trying to kill the Xenomorph. In “Aliens” they are a direct threat and you don’t really have to feel bad at all, it's them or you, but you can try to predict their behaviour more. They are closer to animals than a pure monster like in “Alien”. In Starship Troopers you veer(hoeven) straight into “make-a-point-land” whether you want to or not.
If your Undead are demons who seek to unmake reality possessing the corpses of the dead as imitations of who they were in life, then it's Ok to kill them. Once you start adding nuance and tragic backstories to them, humanizing them... they start just being people with a condition.
If you make Orcs in your world biological creatures with culture then that culture is, by sheer numbers, going to have an uncomfortable similarity to at least one culture in at least one era in human history. A good piece of advice from my early writing at LotFP was that if you could use humans in the adventure to just use humans. The corollary to that is if it would feel really racist to just use humans then it probably is at least sketch as fuck and you should make inhuman monsters. Inhuman is a key term there, More like a Xenomorph or Triffid and not Star Trek prosthetic forehead aliens like the Ferengi.