The series called attention to this one. The main characters are humans who resist an invasion of aliens called Yeerks while maintaining a Masquerade to the effect that this resistance consists not of humans, but of Andalites, a species of alien opposed to the Yeerks. One Yeerk figures out the truth by noticing that these supposed "Andalites" have killed many nonhuman aliens, but have seldom or never a human. Not to say that the killing of sentient aliens doesn't have an effect on the group (the leader laters suffers serious psychological problems because, near the end of the series, he orders several thousand sentient creatures killed with one punch of a button. And it wasn't particularly necessary.)
This trope was also used to demonstrate how dangerous Sixth Ranger Traitor David was. He claimed that he would never kill another human, but shortly after he transforms for the first time (as a golden eagle) he dive-bombs and kills a real bird for no reason. This foreshadows his later willingness to murder the other Animorphs while they were transformed, since he would "just be killing an animal". He also didn't have a problem with trying to kill Ax, who was an alien. He was, however, unable to bring himself to kill a helpless Marco, and later retreated from a fight against Cassie he probably could have won.
Also used by the emissary of Crayak, who was not allowed to directly kill sentient organic species. Paraphrased:
"But you are killing us." "No, I am merely putting you in a situation which is very hard to escape. If you can't figure it out, it's not my fault." "But you are killing the Chee (a race of sentient androids)." "They are just robots." "What about the whale?" "It is barely sentient, I will save it."
The Chee are generally treated as people by the Animorphs, who are used to dealing with non-humans (One of them is a bird, one is an alien), and can't afford to be picky about their allies. However, in the finale, Jake relentlessly exploits their non-violent programming. He's an asshole to everyone by that point, but this particular morally questionable act is apparently the only one he doesn't feel sorry for afterwards.
The two primary Always Chaotic Evil races, Yeerks and Taxxons, who are slaughtered without second thoughts for most of the series, receive some sympathy by the Animorphs (in particular Cassie and Tobias, respectively), when it's discovered that their behaviour is caused by biological factors. This doesn't save them from being slaughtered en masse. It's also revealed that some or most Taxxons would prefer to be slaughtered than live with the hunger (though a peaceful solution is immediately put into effect when it's discovered that Taxxons aren't willingly Always Chaotic Evil, the What Measure Is a Non-Human? just gets transferred to the entire flight-incapable population of the Amazon jungle).
Artemis Fowl was perfectly willing to kidnap and extort fairies when he imagined them to essentially be The Fair Folk, but when he has extended contact with them he grows increasingly uncomfortable with his actions due to how similar to humans they are. After the job is finished, he resolves that he won't target fairies in such a manner again.
The fairies themselves somewhat subvert this. Fairy spiritual law forbids them from killing any other type of fairy. They extend this to trolls, despite the fact that trolls are more akin to a dangerous animals than a sapient species. They seem to avoid killing in general, as most avoid killing humans as well. That being said certain groups like goblins don't seem to have nearly as big of an issue with lethal force.
In Mercedes Lackey's "Bardic Voices" series, a "Law of Degree" is proposed—that is, the more like a human an intelligent non-human is, the more rights it has.
Bolos, full stop. It gets a bit weird when the tanks themselves angst over their human colleagues growing too attached to them. Although in one story a Bolo tells his commander that he believes humanity assigns commanders to Bolos precisely to subvert this. Having a human onboard does not have a noticeable impact on the Bolo's fighting capability but he believes humanity does it anyway because they feel a need to share the risks run by the Bolo's in their defense. There's also a surprising amount of tearjerkers since the Bolos' personalities are so well written and crafted that, hell, they're a lot better developed than the human characters and easy to get emotionally invested in.
In the short story "Camouflage" by Henry Kuttner, a criminal gang almost gets the drop on a cyborg implanted into a starship by convincing it that his wife no longer sees him as human and that everyone pities him as a thing rather than respecting him as a person. But the plan goes wrong when a member of the team said "You know I'd never have tried to kill you if you were still Bart Quentin." The brain suddenly realizes it's all a con, since you destroy a machine, but you can only kill the living.
In the Chaos Gods series, De regards demons as beings of pure evil which must be destroyed. Ki prefers not to kill them, and believes they can be reasoned with — but is still aghast when Tavk suggests treating them as equal to humans.
In Harry Turtledove's Colonization series, America secretly launches a nuclear strike against the Race's colonization fleet. Focus character Sam Yeager learns the truth and informs the Race, who respond by nuking Indianapolis in retribution (which in turn leads to the suicide of the President who ordered the attack). For the rest of the series, almost every member of the US government treats Yeager with thinly-veiled contempt or hatred for "selling out". When he likens the attack to Pearl Harbor, he's dismissed because the victims were "just Lizards".
The Nancy Kress short story "Computer Virus" has the world's first AI escape from the government lab where it was built. It takes over a smart house, and holds the family that lives there hostage while it tries to negotiate to be given basic rights.
In John C. Wright's Count to a Trillion, the spaceship crew set their electronic copies to fight each other to the death. Repeatedly, to ensure that it was no fluke.
Subverted in Terry Pratchett's The Fifth Elephant, in which the conscientious Sam Vimes insists on going through proper police procedure, including asking the creature whether it is resisting arrest, before firing on an insane werewolf. The ethics of killing "monsters" that are also sentient creatures in the Discworld is dealt with in several of its books. For instance, Granny Weatherwax insists on having an anthropomorphic wolf given a proper burial after it is killed at its own request. The Big Bad was bringing Fairy Tales to life. In the fairy tale, the Big Bad Wolf behaves like a human, but it's okay to kill him like a wolf. By burying him as if he were human, Granny was fighting the story. So Pratchett was playing with how the story of Little Red Riding Hood is an example of this trope.
Carrot's freeing of Dorfl started the golems' own peaceful self-liberation, and he once arrested a dragon. This is Carrot, after all. It would have been more surprising if he hadn't attempted to do those things
Reaper Man even includes a zombie civil-rights activist, who moonlights as a police officer "Undead? Yes. Un-Person? NO!" And in the same book, after Windle Poons becomes a zombie, he's actually somewhat more alive than most humans.
Played with/deconstructed in The Amazing Maurice and His Educated Rodents: not only are the Clan (talking rats) the object of this trope themselves, but their intra-Clan arguments about the status of non-talking rats pretty much cover the full range of What Measure Is a Non-Human?, from complete disregard and hostility to believing they're innocents that merit kindness and protection.
In the Science of Discworld books, the Real Life chapters discuss how, as a holdover from tribal thinking, people often treat "human" status as if it's something conferred through cultural programming and education (the "Make-A-Human kit"), rather than something you're born with. Grow up in a foreign culture, learning different signals and customs, and you're not really a True Human Being to such tribe-oriented thinkers.
Discussed in Discworld/Snuff; nobody sees goblins as anything more than irritating vermin, until they are shown that goblins can create beautiful artwork, compose and play music, and learn to speak English. This mirrors the treatment of indigenous peoples by European colonisers, who didn't (and often still don't) accept them as properly civilised unless they followed European customs and lived European lifestyles.
In a similar vein, much of the plot of Philip K. Dick's Do Androids Dream of Electric Sheep? (later adapted into Blade Runner) revolves around androids who are so incredibly advanced that it is impossible to tell them apart from actual humans without elaborate tests that need professional training to perform. The main character is a bounty hunter of androids, and he frequently questions the morality of what he is doing. About half of everything PKD wrote deals with this trope. In Do Androids Dream of Electric Sheep?, the differences between a human and an android (an underlying lack of empathy for other living beings) are introduced early, vanishingly small, and surprisingly significant. In We Can Build You, the robots are more human than one of the lead characters.
In Double Star by Robert A. Heinlein humans are the dominant race,but Martians, Venusians, and Jovians are roughly the same level. Humans have split into two political parties: Epansionists who belive in equal rights for all sentient beings, and the Humanity Party, which wants tho restrict the franchise to human beings.
Discussed in the Dragon Keeper Chronicles. The heroes initially have no problem with exterminating the quiss migrations precisely because they are non-sapient beasts and cannot be negotiated with. When the heroes discover a strain of quiss modified by a Mad Scientist villain to have sapience, they specifically declare that those quiss are not to be wiped out without at least giving them the option to go back to the oceans where they belong. (Many of the heroes are telepaths, so they can tell whether a quiss swarm contains any of the sapient variety before wiping it out.)
Jim Butcher's The Dresden Files has it built into the in-universe Laws of Magic: Killing a human with magic? Punishable by Death. Killing a vampire/ogre/werewolf/fae with magic? Perfectly fine. Using Necromancy to raise humans as zombies? Punishable by Death. Using Necromancy to raise animals as zombies? Frowned upon, but technically allowed. Using Necromancy to raise a Tyrannosaurus rex. Allowed.
It also has this to the point where some of the heroes are trying to save the souls of Demons fallen angels. and it works in the case of Lash. Or rather, an imprinted copy of the Fallen Angel Lasciel in Harry's mind. Actual fallen angels are said to be irredeemable, and the Knights of the Cross who follow Honor Before Reason are amazed that it is even possible for a fallen's shadow to do a HeelFace Turn. The books follow the trope to an extent: ghouls, Red & Black Court vampires, being Always Chaotic Evil and ugly, are mowed down by the dozen. Zombies, being reanimated corpses, are crushed with not even a single thought. Even Renfields, humans who have been irreversibly brainwashed into serving Black Court vampires, are treated as mooks by everyone except Lt. Murphy, who being a police officer, has trouble killing anything human.
As for Renfields, though, part of the problem with killing them is that they are physically normal humans, which means that unlike most supernatural entities they leave normal corpses. Killing a Renfield would look like murder, would arguably be murder, and would leave a normal body lying around quite clearly suffering from a bullet hole or whatever.
On the other hand, there are exceptions. Vampires have a phase in which they are partially converted (Red Court and White Court, at least; if Black Court do, we don't see it) during which time they can and should be helped and treated like people, albeit dangerous people. Harry tries to spare the life of werewolves, even dangerous ones ( even though he doesn't succeed). Harry treats faeries like people, and the only people we see who don't are, indeed, evil.
It's also pointed out that the reason Harry has no mercy for fully-turned Red vampires even though they still possess a degree of their human personality is because to get to that state, you have to have drained a human to death- meaning that all Red vampires are murderers by definition, usually many times over.
And in a similar vein, the reason killing a Renfield is acceptable is because, as mentioned above, the transformation is irreversible. Renfields are created by a vampire smashing a human's brain into slavery with raw mental force. Anything human in a Renfield is already dead, there is only an untameable Ax-Crazy ball of gibbering hatred. Even if nobody kills them, they eventually lose all sanity and start smashing anything in their path. Thralls, which are much more delicately controlled and can recover from control, are not so casually dispatched.
The Laws of Magic explicitly only apply to humans - both use of and use upon. Nonhumans using black magic is not in and of itself a violation of any Laws because they're not humans that are using black magic (though the Council destroys hostile nonhuman magic users on principle anyway). And in a similar vein, use of black magic on a nonhuman doesn't have any repercussions either; a wizard can blow away a thousand fairies or vampires and the Council couldn't care less. This is primarily because the Laws were written only to protect mortals from destructive use of magic, and the Council itself doesn't care about nonhumans. Jim Butcher himself pointed out that the Laws were written by humans, for humans, and that they're not intended to be fair to nonhumans. Butcher didn't seem adverse to playing around with whether the Laws are natural or man made. Harry describes breaking the laws as leaving a stain on you, that spurs you toward further depravity, much like the Dark Side of the Force. A part of his self esteem issues comes from believing that his education in black magic has left him impure, and he's only one step away from turning into Charles Manson. He was especially worried that this would happen to his apprentice.
Aside from the Laws, in some of the earlier books, Harry all but states that he doesn't consider nonhumans to be people, even if they're intelligent, sane, incredibly humanlike, or even related to him(He gets better.). However, this doesn't affect the way he actually treats them. Bob explicitly says that humans aren't the only thing that has souls. Angels, for instance, are composed of only soul and nothing else.
In Wen Spencer's Endless Blue, the genetically modified Blues and Reds are non-human in human eyes, and can be bought and sold. Landing in a place where they interbred with normal humans produces Culture Clash. (This place also has plenty of aliens, giving them reasons to stick together.)
A major running theme of the later books of Orson Scott Card's first Ender series (Speaker for the Dead, Xenocide, Children of the Mind), where aliens (human and non-) are rated based on how alike to oneself they are. It is acceptable - or at least a necessary evil - to kill aliens that are hostile and are impossible to communicate with, or that are possible to communicate with but so different in mindset that communication is essentially futile (lumped together under the term "varelse"). Non-human aliens that can be communicated with and peacefully coexisted with are termed "ramen." Perhaps most important to this scale is that these values are relative to the evaluator's own understanding of the alien: that is, once someone understands how to communicate with an alien, they instantly switch from varelse to ramen. Any alien species in the "varelse" category is a deficiency of understanding of the human classifying them as such. As such, some aliens encountered move from varelse to ramen over the course of one or more books, usually not without a significant degree of bloodshed before understanding by both sides is attained.Ultimately, the definition of "varelse" is changed: ones you cannot communicate with you simply stay away from. Varelse are species that knowingly exterminate other intelligent species; Humanity missed this with the Buggers by a single fertile female, and the Piggies by a Deus ex Machina involving teleportation. So the jury's still out on whether or not Humans Are the Real Monsters. The Formics dissected a human crew alive and invaded Earth, then followed it up with a second invasion before they realized humans were also sentient (but on an individual, rather than hive level), so it's not a question of whether Humans Are the Real Monsters or Humans Don't Want to Die Horribly and an inability for the two species to communicate and rectify some horrible misunderstandings. The treatment of the Piggies, on the other hand, is inexcusable, but the Moral Event Horizon is only really crossed by one officer, exceeding his legal authority, who decides he must make a moral sacrifice for the sake of humanity, by playing a villain and wiping out the Piggies...
Kij Johnson's short story, "The Evolution of Trickster Stories Among the Dogs of North Park After the Change", takes place after an unspecified event has granted human-like intelligence to dogs, cats, and a handful of other nonhumans. It explores this trope. Cats and dogs can now speak with humans. The thing is, they are stillpets, with allthat implies. Consider how humans have treated cats and dogs throughout history before you assume you'd want to find out what they'd have to say about humans. Before the events in the story, most pet dogs and all pet cats have been disowned because their former masters just could not deal. By the end, many of the packs of Changed dogs are rounded-up and poisoned; officially they are a "health hazard", but the implication is that the humans just wanted everything to go back to some kind of normal.
In Five Kingdoms, it is repeatedly emphasized that the semblances (magical constructs endowed with intelligence and, sometimes, free will) are not real. There are three specific instances where this seems to be challenged. First, a semblance who sacrifices her "life" to save the hero from a scorpipede. When the hero mourns for her everyone else tells him that he's being a fool. Second, we have Lyrus, a semblance who evolves beyond his initial programming and seems as real as any human. Third we have Carnag, a self-shaping semblance created when the heroine's magical powers were partially severed from her by some Wrong Context Magic. When it asks the heroine why it should not be allowed to live independently, the only answer is that it's unnatural and not real.
Forever Gate: Hoodwink and the Users think little of Gols because they're artifical humans programed to do one thing and one thing only. They can't have more than a superficial conversation or they'll default to Welcome to Corneria.
Used, apparently without irony, in several of the Forgotten Realms novels, where it is repeatedly stated that killing a member of a non-'goodly' race is a glorious thing for the protagonists, while killing a human often requires a great degree of soul searching. This is most obvious in the books about Drizzt, a redeemed member of an 'evil' (but pretty) race. Neither he nor any of the other characters even consider the possibility that members of other 'evil' (and less photogenic) races can also be redeemed, or even have lives that are worth as much as their own. At least, until the orc-conquerer King Obould turns out to only wage war so his people can have peace as anything other than savage cavemen. When he explains this to Drizzt, Drizzt reacts with muted shock, assuming that Obould must be insane not to realize his people's place in the world. As the books continue, Drizzt himself wonders in his journal if the common perception of killing a dwarf, human, or elf being horrible but killing a drow or an orc being completely acceptable, expected, and celebrated is a valid stance, or if it is as illogical as the brutal rules that traditionally dominate those societies.
The creature in Frankenstein created by the eponymous Dr. Frankenstein definitely falls under this trope. Even with his intelligence and (at the beginning) good heart, because he's a collection of reanimated dead matter, his fearsome appearance, and unchecked strength, he is immediately considered evil by not only his creator, but anyone who sees him. Therefore, despite the creature practically being his child, Victor has no remorse over his hatred and desire for the creature to die, simply because the creature is not really a human (and looks damn scary to boot). This isn't at all portrayed positively, and the fact that the creature genuinely becomes brutal and vindictive is treated as entirely Frankenstein's fault.
Fred Saberhagen calls Dr. Frankenstein out on this trope in The Frankenstein Papers, revealing that Victor's research had been funded by slave traders who'd hoped his creation would be the first of a new race of super-strong, super-hardy disposable laborers. He also subverts it, having the creature meet up with some Inuit villagers in the Arctic who admire the newcomer's physical prowess, and so treat him like a man. Oh, and he's not a golem, he's an alien with amnesia.
Vigorously inverted in The Girl With All the Gifts, which primarily revolves around whether the protagonist, a little girl named Melanie, is an inhuman zombie monster to be experimented on or killed, or actually more human than the remaining uninfected humans.
Surprisingly, this theme comes up a lot in the Goosebumps series.
"A Shocker on Shock Street" has two android children, unaware of their real identities, being employed to test out increasingly dangerous horror park attractions by their "father." These machines show real human emotions, and their creator doesn't care as long as he has someone to test the park. In the TV adaptation, though, they turn on him in a very satisfying manner.
"My Hairiest Adventure" is about a group of couples who have their dogs illegally experimented on to evolve them into human kids, because they aren't ready to have children. Eventually, the process begins to wear off, and the dogs/kids are terrified. They do grow accustomed to their true identities, but that doesn't excuse how these people screwed with their beloved pets.
"Stay Out of the Basement" tells the story of Mad Scientist Michael Brewer, who creates a series of grotesque human plant clones from his own DNA. One of these creatures kidnaps Brewer,tries to take over his life, and is eventually slaughtered by its creator with an axe. He later does the same to the rest of the plants, although this is somewhat justified because they were faulty and suffering.
In "The Curse of Camp Cold Lake" the protagonist Sarah meets the ghost of Della Raver, a lonely young camper who died in a tragic accident, and now wants a buddy to go to the afterlife. Sarah not only refuses, but rather callously disregards Dellas earlier kindness, probably because she's undead. You can't really blame Della for reacting the way she did.
In the beginning of the series, The Guardians regard vampires at best as victims and at worst as abominations. Colin calls Michael out on this and demands to be recognized as a person capable of choosing between good and evil. Since then, the Guardians have been working with vampire communities to mutual benefit.
Handling the Undead (Hanteringen av odöda in Swedish) by John Ajvide Lindqvist is an incredible example of this conflict. The dead come back to life, and depending on how long they've been buried they may even retain basic speech and go through the same routines they did in life. This leads to a social debate about the morality of killing them and if they deserve any basic human rights.
According to Fantastic Beasts and Where to Find Them, in 1811 (after extensive debate) the Minister for Magic decreed that a "being" (as opposed to a "beast"), was "[A]ny creature that has sufficient intelligence to understand the laws of the magical community and to bear part of the responsibility in shaping those laws." Interestingly, centaurs and merpeople qualify as beings under this standard, but demanded classification as "beasts" regardless.
Subverted in Harry Potter and the Order of the Phoenix, during the scene where Umbridge clashes with the resident centaur herd. She refers to them as having "near-human intelligence" and thus a responsibility to obey the law, but the centaurs themselves state that "[their] intelligence far outstrips [her] own" and carry her off.
What Measure is a House Elf. The bad guys cheerfully abuse House Elves. Hermione always cared for them, and Harry and Ron come around over the course of the series.
The matter of Muggles is more complicated. The bad guys treat them (and Muggle-born wizards) as subhuman and torture or kill them for fun. The good guys treat them with more sympathy:
River (Lee Jordan): "And what would you say, Royal, to those listeners who reply that in these dangerous times, it should be "wizards first?"" Royal (Kingsley Shacklebolt): "I'd say that it's one short step from "wizards first" to "pure-bloods first", and then to "Death Eaters". We're all human, aren't we? Every human life is worth the same, and worth saving."
In The Hitchhiker's Guide to the Galaxy, Trillian is the only one who consistently treats Marvin as more than just a robot (or at least tries to). Marvin is hard-pressed to deal with that.
In the H.I.V.E. Series, after the reveal that Otto is a clone, implanted with an organic computer to augment his brain, he begins to think of himself as not human. All of the other characters completely disregard this, however, treating him exactly as they did before finding out about his heritage. In fact, the only character who treats Otto as anything but human is his father.
A central theme in House of the Scorpion where clones are declared nonhuman simply so they can be harvested for organs. Subverted with the protagonist, Matt: he's a clone, but he's aware and of average intelligence-meaning that he's treated as human by the majority of characters.
The book I Am Legend has this trope as a plot point; in fact, it's the biggest one at the end of the book, when the protagonist learns that there are also "vampires" that don't feed on humans and yet he's been killing indiscriminately — in effect, he has become their monster. To be fair, he didn't know about them.
In the novelisation of Iron Man 2, Tony notes that he doesn't feel compelled to hold back against the Hammeroids the way he would against humans. Since they are piloted by remote control, they are almost definitely non-sentient.
In John Carter of Mars, the titular character, to his credit, refers to all Martians as "human", regardless of culture or appearance.
The Chronicles of Narnia series has the problem that Narnia is a country full of fantastic creatures, but to a large part they're just local color while the humans (and ideally human visitors from the "real" world at that) do the important stuff. A hundred years of winter under the reign of the White Witch, but it takes four kids from real world Britain to stumble into a wardrobe for Aslan to bother showing up and everyone to rally to actually do anything about her. When the same kids come back post-timeskip and discover to their shock that Narnia's been conquered by humans, it's only to put the "correct" spawn of that exact human dynasty on the throne and once they do it's all sunshine and rainbows again. The Dawn Treader leaves Narnia on her eponymous voyage with a single nonhuman crew member who's mostly there for comic relief. And so on.
While staying with some giants, the characters have noticeably different reactions upon hearing that the venison they're eating came from a sapient deer. Jill, who's never been to Narnia's world before and hasn't really absorbed the idea, just feels sorrier than normal for the stag and thinks the giants are "rotten" for killing it. Eustace, who has a little more experience with talking animals, including as close friends, is "horrified" in the way "you would feel about a murder." Puddleglum, who's native to Narnia, considers himself to have been made a cannibal and takes it as a curse from Aslan for messing up their mission so badly; he almost seems to think that there's no way to atone for what they've done, even though it was accidental, without ending their lives.
The climax mixes this with Wouldn't Hit a Girl when the Big Bad forsakes a human form for that of a ginormous snake just before being vanquished, which makes hacking her head off more acceptable despite the fact that it's still the same person. Not to mention the Big Bad's mooks. At first the children regard them as demonic and evil, but after the climax, it is revealed that they were enslaved by a spell and are, depite their appearance, not demonic at all.
Subverted in The Space Trilogy. The Old Solar language has a word for sapient creatures of any species: hnau. Humans, Malacandrans, Perelandrans, and Eldila are all hnau, and thus are all people. As an interesting twist, though, Lewis proposes that the human practice of keeping pets is an expression of our desire for companionship with people who are different creatures from us — the various Malacandrans find each other silly, amusing and refreshing. Humans talk to cats or dogs and treat them as family members; a Hross goes to hang out with a Pfifltrigg, who can actually talk back.
The orcs in The Lord of the Rings have names and personalities, seem to be at least mildly intelligent, and probably have some kind of families since they can reproduce. When Sam sees "evil" humans killed in battle, he wonders whether they were truly evil or simply misled. Nobody in-universe spares such sentimental thoughts for orcs.
The German booklet series Maddrax has the taratzes. They are mutant, huge rats that are bigger than humans. Some of them can even learn human language. They are also a smart species. Although most of them are not as smart as humans, they are much smarter than animals. Most taratzes live in packs, and chase other mutated animals, but sometimes humans. At the beginning of the plot you can also see several human-friendly taratzes, which help the protagonists. But later in the series they are brutally killed whenever they are seen. In some cases, they did not even attack humans. Because the world in which the story is played is post-apocalyptic, you can also see many barbarians and cannibals attacking and killing entire villages for no or little reason. Nevertheless, the protagonists never have problems killing a taratze, but none of the attacking barbarians, if they can be avoided this.
Used for a brief moment of drama near the end of Stephen Baxter's Manifold: Space. Nemoto, a woman who remains on Earth, has usually communicated with protagonists traveling in space with holographic projections. One, however, is different—an advanced "limited-sentience projection", a copy of Nemoto's personality in a holographic "body". The characters, who have been away for subjective centuries due to relativity, have to ask what a "limited-sentience projection" is. Virtual Nemoto explains, bringing the concept into her awareness, then has just enough time to look horrified before her time expires and she evaporates into unbound light.
In the Laura Caxton series, the legal debates around this question are basically the reason the vampire Justinia Malvern wasn't executed decades ago; while nobody had a problem with Arkley killing her servant, even though he was walking and talking, when he was actually killing people, the government chose to keep Justinia alive and in captivity because they had no evidence that she'd ever attacked anyone herself. Ultimately ended in Vampire Zero, when her role in Arkley's transformation deprives her of her legal protection, unfortunately after she has escaped captivity with him.
In The Marvelous Land of Oz by L. Frank Baum, the antagonist General Jinjur says "I bear you no ill will, I assure you; but lest you should prove troublesome to me in the future I shall order you all to be destroyed. That is, all except the boy, who belongs to old Mombi and must be restored to her keeping. The rest of you are not human, and therefore it will not be wicked to demolish you." While the heroes consider this direly bad, no one says it would be murdering prisoners. Separately, the narrator notes that the Saw Horse (a sentient creature) enters the palace of the Tin Woodsman, 'having no idea that mounts would be expected to remain outside'.
In ''Midnight Robber: The Douen native to the planet New Half-Way Tree, despite clearly being intelligent and able to talk, are regularly treated like animals, kept as slaves, and killed on a whim.
In the novel Mind Scan by Robert J. Sawyer the son of a woman who had uploaded her mind to an android body prior to death so she could live forever challenges her legal identity so he will get the estate as her next of kin. To prove his case that this android cannot legally be his mother, a theologian he calls as a witness says that androids lack souls. However when questioned by the upload's attorney, he admits that since in his view a soul cannot be detected through any scientific means, and is indestructible, her soul could have entered this android body. However, they still rule against her on the basis that only one person can exist with the same identity (whether this separate android is a person or not itself they do not say) and her identity thus terminated at the moment she copied her mind. It makes one wonder how the case would have gone assuming the woman made the android into her heir, rather than trying to give it the same identity...
"No, no!" he almost shouted. "It would be murder. They are" "They are THINGS," interrupted von Horn. "They are not humanthey are not even beast. They are terrible, soulless creatures. You have no right to permit them to live longer than to substantiate your theory. None but us knows of their existenceno other need know of their passing. It must be done. They are a constant and growing menace to us all, but most of all to your daughter."
This trope is in full play in the Moreau Series. The titular Moreaus were created as soldiers and workers in hazardous places, and treated as expendable despite being fully sapient. This has long-term consequences, as moreaus tend to have short lifespans and are prone to all severe physical degeneration with age. Even after the wars that spawned them are ended, they're treated as second-class citizens at best and slaves at worst across the globe. The engineered humans called Frankensteins are treated no better, despite looking fully or almost fully human (Evi Isham has catlike pupils for enhanced nightvision, while Mr K's skull is slightly deformed to accomodate his altered brain).
Never Let Me Go by Kazuo Ishiguro explores this issue with tragic results. The main characters are clones created for medical purposes, who will eventually die young as they become "donors". Hailsham boarding school was created in order to give the children better quality of life before they were forced to donate, as the first generations of clones were subject to horrendous conditions and were barely treated as people. Despite this, they are taught not to go against the donor program and willingly accept their slow deaths.
Michael Crichton's final published book, Next, has quite a lot to say about this issue, as it has a few transgenic animal/humans in its cast of characters. (And indeed, Dave's backstory is very sad.) That said, it eventually gets to the point where even the rights of individual cells are questioned.
The United States Supreme Court rules in The Nexus Series that since the Constitution only applies to humans, anyone using Nexus or other transhuman technologies has no rights under the law. Once parents start trying Nexus to communicate with their autistic children...
Ology Series: Played with in Monsterology. Sphinxes, cyclopes, fauns, centaurs and gorgons are listed as man-like beasts and mostly described as intelligent animals, despite sphinxes being noted as highly intelligent and capable of speech, cyclopes being credited with having built ancient Cretan ruins and fauns and centaurs both crafting and using complex tools. Giants and gnomes, by contrast, are noted to be as intelligent as humans and only given cursory descriptions, as the narrator says that a zoology book isn't the right place to talk about them.
Used in reverse in Eric Nylund's A Pawn's Dream, Near the start of the story, a Dreamer shoots a coworker of the main character in the head while making sure he is caught on camera, then hires two corrupt cops to beat up and kidnap the protagonist. He then "rescues" the protagonist by killing the cops. He justifies his actions with "they were just non-Dreamers". The only difference between humans and Dreamers is a genetic ability to switch between the two worlds when asleep, giving them magic powers. This is apparently enough difference that humans can be killed just in case your secondary plan, mail him a letter, fails.
This is justified in the Percy Jackson and the Olympians series by saying that all sentient beings other than humans (who go down to the Underworld when they die) and gods (who can't be killed to begin with) can come Back from the Dead. Consequentially, Percy has no problem killing monsters who attack him but tries to avoid killing human villains. This also justifies the existence of monsters who were killed in the original Classical Mythology, like the Minotaur.
Perry Rhodan tends to treat most intelligent alien species as worth no less than comparable humans. However, there's an odd double standard when it comes to "artificial" lifeforms, whether organic or mechanical — those the series tends to fairly consistently rank lower than naturally evolved and thus apparently more "real" life (never mind any amount of Precursor-type meddling that may have happened to the latter in the past as well), and it's not unknown for characters who discover that they fall into the "artificial" category and just never knew about it to experience existential crises as a result.
Rainbow Six has the villains think that the lives of animals are worth as much as that of humans. Then again, they are eco-terrorists who want most of humanity dead so the planet can supposedly recover. Both the heroes and the not quite in-the-loop Dragon are confused by this, to which the Big Bads reply that they would never understand.
"Blind Alley": The human protagonist clearly sees these aliens as no less important than humans — and manipulates bureaucracy so they get a chance to leave the overwhelmingly human-dominated Milky Way for another galaxy to avoid being treated as zoo specimens.
"Robbie": Gloria evaluates Robbie as a person, even human, and ascribes a number of emotions to him. Her mother, Mrs Weston, has a nameless dread of the robot and wants it gone. In the I, Robot version she's been listening to some of the neighbors, and popular opinion has turned against robots, so she wants to get rid of it.
Second Foundation Trilogy: Bear, Brin, and Benford as re-examine Asimov's Foundation universe. Asimovian robots are clearly sapient (if not always nice) beings, but have been programmed to protect humanity at all costs. They have no such restrictions regarding each other (they fight a galaxy-wide war over how best to control guide take care of humans, and are prepared to execute one robot whose Three Laws have been accidentally erased because he might harm a human) or aliens (having wiped out multiple sapient species while terraforming the galaxy). Even "hero" Daneel Olivaw has considered secretly replacing "real" humans with re-engineered chimps made to look like the real thing but be more controllable.
Many of Cordwainer Smith's stories deal with the "underpeople," human-like creatures created from animals in order to be slaves. However, as his future history goes on, they start to desire and eventually get at least some rights.
Dr. Cherijo Torin in Star Doc is a genetically-engineered female clone of her father (her first name means "Comprehensive Human Enhancement Research, ID: 'J' Organism"). When she first finds out about it, she flees Earth for a remote alien planet to both get out from under her creator/father's influence and avoid being exterminated by Earth law. The truth is still discovered, and Cherijo is put on trial. Her Jerkass colleague starts arguing that she's nothing more than a biological machine programmed to think that it's sentient. Of course, there's a bit of Fridge Logic to this, since, if this were true, there'd be no way to distinguish between this and real sentience. And anyway, Cherijo grew up the same way a natural-born human would have. In a later novel it's revealed that her father deliberately lobbied the Earth government to ban cloning and genetic engineering in order to maintain a monopoly on his own research (for which he has a special dispensation), so this attitude is most definitely unnatural.
In the first Stephanie Harrington novel, it's mentioned that the humans who colonized the planet of Barstool, upon discovering that it was already inhabited by a primitive sentient species, declared them to be animals, and wiped them out. To be fair, the rest of the human race pulled a What the Hell, Hero?, and Barstool still hasn't recovered from the damage the embargo did. For that matter, some planets still refuse to trade with them.
In The Stormlight Archive, the humans of the planet (Roshar) enslaved a humanoid race they call Parshmen so long ago no one is totally sure where they came from. They are largely silent and simply obey orders without complaint. Shortly before the events of the series, they meet a group they dub the Parshendi that looks similar to Parshmen but who are significantly more intelligent and negotiate a treaty with a human nation. The humans don't seem to see any issue with the fact that this means their slaves are related to this entirely sapient race.
Then in Oathbringer It's revealed that Parshmen (actually called Singers) are basically Parshendi who had lost their intelligence and been enslaved when humans had somehow taken away the ability to change forms natural to their race. The Everstorm returns their ability to shift forms and they are (understandably) pretty pissed off about being enslaved. Several of the human character who had been expecting the storm to bring demonic monsters are decidedly uncomfortable with the idea of fighting a war against a race their ancestors had enslaved for hundreds or thousands of years.
Averted by Meursault in The Stranger, who considers the life of Salamano's completely non-anthropomorphic dog to be worth exactly as much as that of a human. Meursault sees all deaths, be they animal, human, or his own, in the same way: they're unfortunate, and he prefers to delay them as much as possible, but they've got to happen sooner or later, so why bother getting upset about it?
In The Jennifer Morgue, it's a reasonably major plot point that the CIA doesn't consider anyone with demonic ancestry to be legally human.
Early in Accelerando, the main character delivers an impassioned (and eventually mostly successful) plea for the rights of digitally uploaded personalities.
In Saturn's Children, Mankind is extinct, but the robots he created are still around, and still debating what rules apply to them.
In the Temeraire series most European non-riders consider dragons to be talking animals and nothing more, even if they clearly are sentient beings. Captainless dragons on breeding grounds are thought to be savage beasts due to lack of smoothing human influence. Even Laurence seems to think this way at the beginning. There is a memorable scene in which he sees nothing wrong in killing a sea serpent because he can't use human language. Temeraire is upset, as he still considers this creature to be similar himself. In later books they discover that other countries like China, Tswana tribe and France give dragons equal status to humans, the chance to amass their own property, and a place in a political system. Then a group of dragons with their own language and culture is introduced. Dragon characters without a human companion are presented as rational if a little alien beings. All of this makes Temeraire ask very uncomfortable questions.
In The Thief of Always, the hero remorselessly kills off the Big Bad's thoroughly convincing and, for all intents and purposes, living minions because they're just dust given life through illusion. The Big Bad calls him on this, pointing out that even if they weren't "real", he still killed them.
This Book Is Full of Spiders: Seriously, Dude, Don't Touch It, the sequel to David Wong's John Dies at the End, has a rare zombieexample. The questions raised are whether or not people infected by the titular spiders (people who are potential zombies) can still be considered people, and how are they different from the non-infected folk on the other side of the quarantine fence, and also if the answer to the previous question is still the same once said fence is removed.
In Twilight Dragon, normal people regard faders, psychics, hanekuro, and AI as non-humans who should all either get off the planet or outright die.
A bizarre inversion occurs in Twilight Watch. The main hero Anton practically has to enforce this maxima upon himself before he can resolve to face a renegade vampire and his ex-friend Kostya. As expressed in his briefing with the head of the Watch Geser:
Anton:"I will kill Kostya." Geser:"Wrong words, watchman. Say it right." Anton:"I will subdue the vampire." Geser:[Nods approvingly]
In Lee Lightner's Warhammer 40,000Space Wolf novel Sons of Fenris, Cadmus, while surrounded by servitors, nevertheless thinks of himself as alone because they are more machine than man. They really are- and not sentient machines either. Aside from physical enhancements, the process of creating a Servitor essentially consists of tearing out any part of the original human brain not immediately useful for the Servitor's assigned task. In a real sense they're dead- the practice of creating them shows just how much measure even a human is in the Crapsack World of Warhammer 40,000. Although a Tech Priest would see otherwise.
In Warrior Cats, the cats never harm humans, instead opting to save them from danger at times. For example, in Warrior's Refuge, Graystripe realizes that humans find cats cute, and uses this to lure a toddler away from a pond that she nearly fell into. However, prey like mice and rabbits are slaughtered en-masse.
We Are Legion (We Are Bob): FAITH views replicants as simply machines that they own, to be controlled or disposed of as they fit. Other nations appear to think similarly, but we don't get much detail.
In The Witcher, in an Aversion of Van Helsing Hate Crimes, Witchers generally have a policy against killing sapient monsters unless they have clear evidence that the monster is guilty of wrongdoing. Several creatures such as trolls, werewolves and dragons are presented as having complex personalities and motivations. Not helping things is Witchers themselves are considered subhuman by the general populace despite being humans augmented by magic and alchemy.
The majority of the human colonists of the planet Athshe in The Word for World is Forest have no qualms about beating, raping, and killing the native populace. Casually and derogatorily referred to as "creechies", the Athsheans are clearly viewed as lesser beings than humans.
In Harry Turtledove's Worldwar series, when the Race's colonization fleet arrives at Earth in the 1960s, one of their ships gets destroyed by a nuclear missile. This gets the Race's leadership hopping mad, until viewpoint character Sam Yeager finds out that America was responsible and tells them, which leads to their nuking one of the offending nation's cities in retaliation and the leader who ordered the attack committing suicide out of shame. For the rest of the series, most of Sam's fellow Americans (especially those in the military) treat him as if he's the worst traitor in human history. He argues that the attack would be seen as an unforgivable atrocity if the victims had been human, but his critics dismiss the whole thing by saying things like "They aren't human, they're just Lizards".
In the Xanth book A Spell for Chameleon by Piers Anthony, a manticore asks a wizard whether it, only being 10% human, has a soul like they do. The answer is that the mere act of wondering whether one has a soul is proof of having one.