Out in the reaches of space, an alien race exists that has developed far beyond any human civilization. Said aliens will believe they are justified in killing or enslaving humans due to their higher intelligence. They don't necessarily hate humans, they just believe that humans are so insignificant as to be unworthy of moral consideration. Bonus points if they draw parallels between the way they treat humans and the way humans treat other animals.
The purpose of this trope is often to question the attitudes that justify the exploitation of animals, the environment, and/or other cultures. Works that use this trope ask the question, "What if there was someone who treated you the way you treat those you have power over?" In particular, this trope often draws inspiration from the white supremacist attitudes that were used to justify slavery, the actions of European colonial empires, and America's westward expansion, to say nothing of the Nazis.
Not every alien species that victimizes humans fits this trope. As a guideline, please note that this trope applies if either:
A character in the work articulates this trope, such as the aliens explicitly stating that their superiority frees them from moral considerations; or
In Wicked City, Makie's ex-lover Jin tries to make her admit this by saying, "Human are lower-class creatures than us. They're only fit for slavery. That's their heritage."
Puella Magi Madoka Magica's Kyubey plays with this trope. He turned vunerable teenage girls into magical girls in order to fight witches, but doesn't tell them that he does so by turning them into Liches. Then the girls find out that if they don't keep their Soul Gem pure, they become witches too, and it then it turns out he's doing all this to collect energy to fight the Heat death of the universe. He justifies it by wanting to prevent said heat death, and by the fact that his kind has been assisting humanity since the stone age. All this while subtly implying that his race regards humanity the way humanity regards cattle. However, Kyubey doesn't have emotions, so he doesn't do this because he thinks he superior to humanity (or at least that's not the most important reason). He does it because they need to prevent the universe ending, and this is the most efficient way to do it.
The comic book version of Cowboys and Aliens gives the aliens this viewpoint (pretty much explicitly stated to be a metaphor for Manifest Destiny and the treatment of Native Americans). There's actually a scene where a white guy screams "they don't have the right to do this to us just because they have better guns!", only to be met with a Death Glare from an Indian.
Which, of course, ignores the fact that most western Native Americans knew full well why the Europeans treated them as enemies—they didn't act any different, ifsowell, in their own conflicts. In most southwestern Native American cultures, all ethics ran on What Measure Is A Nonhuman...and for "human", read "Navajo, Apache, Comanche, Kiowa", etc. The Chiricahua Apache—the main tribe in Cowboys and Aliens—considered only their own branch of the Apache to actually be people; anyone else, including other Apaches (and Navajos, who some Apache also extended "personhood" to) was fair game for raids or, if someone killed one of them while fending off a raid, a punitive massacre (which is what "war" meant to the Apache). Meanwhile, at least half of the Apache in Texas were killed off by the Comanches, who were if anything evencrueler. And so on.
The film Avatar runs on this trope, though it's a rare case of humans being the superior species while the "primitive" Na'vi have to deal with mankind strip-mining their planet.
Similar to the Avatar example, there's the animation Battle for Terra, where the human race stages an invasion of an alien planet. They justify this by the right of their superior technology, their view that the aliens aren't sapient, and that Earth was destroyed and they need to repopulate the species somewhere.
Megatron (and probably the other Decepticons) in Transformers, who says that "Humans don't deserve to live."
The War of the Worlds by H.G. Wells. Unusual in that this is articulated by the human narrator at the beginning of the book. After reflecting on how much more advanced and intelligent the Martians are, he concludes:
And before we judge them too harshly, we must remember what ruthless and utter destruction our own species has wrought, not only upon animals, such as the vanished bison and the dodo, but upon its inferior races. The Tasmanians, in spite of their human likeness, were entirely swept out of existence in a war of extermination waged by European immigrants, in the space of fifty years. Are we such apostles of mercy as to complain if the Martians warred in the same spirit?
Harry Turtledove's Worldwar series. The reptilian Race considers themselves eminently justified in conquering Earth and making humanity a subject race because of what they view as their incomparably superior culture and technology, even though said technology turns out to be not quite that advanced over humankind's. Indeed, one of the "Lizard" characters pretty much lampshades this during a conversation with a human character, when the human points out all the rights and liberties that his people yearns for and the Lizard claims, in all seriousness, that humans would enjoy those freedoms under the rule of the Race.
They also find the Nazi arguments for the latter's claims of being the Master Race lacking. Then again, they don't even try to explain why they think their culture is better. Any time someone asks, they simply say "The answer should be obvious". Many times they express their outrage that humans have a level of technology close to theirs. They have no right to have technology like this by all rules (of course, by their rules, even a small technological change should take centuries of careful integration into society in order not to upset the status quo).
The Strong Races are this to the Weak ones in The Stars Are Cold Toys duology. The galactic rules are like this: if your race is powerful enough to wipe out any other race except fellow Strong ones, you can do whatever you please. If it isn't, you better possess some unique talent useful to the Strong races, or be wiped out by them to make space for new strains of evolution.
Out of the Dark by David Weber, is about a race of aliens who usually do this successfully but get way more than they bargained for with humanity.
In just about any story featuring vampires, the vampires consider themselves to be on top of the food chain, and consider humans their prey.
In Darren Shan's vampire books, the Vampires avert this but the Vampaneze play it straight.
The dragons from Robin Hobb's Realm of the Elderlings are like this. Even after a long absence and teetering on the brink of extinction, they fully expect humanity to serve them.
Robert Westall's Urn Burial: Stated almost word for word by the Wawaka as the reasoning behind their disdain for and lack of concern over, humans. When Ralph accuses them of torturing humans, they respond that humans treat animals in exactly the same way.
The Death Gate Cycle: The Sartan and Patryns each believe that they are the superior species and the only one fit to rule the lesser races of humans, elves and dwarves. The lesser races are viewed as children to be protected by the Sartan and subjects to be ruled by the Patryns and pawns to be sacrificed by both. Inverted in one realm where the lesser races have achieved peace amongst each other and offer to mediate the conflict between the Sartan and the Patryns.
In Star Trek: Millennium, Leej Terrell considers the Bajorans little more than cattle, and refuses to accept that Cardassia was doing anything wrong in enslaving them. Indeed, she tells Sisko that humanity's biggest problem is its refusal to distinguish "truly sapient" races like the Vulcans from "stock" like the Bajorans.
The Shedai in Star Trek: Vanguard believe they rule other species by right and generally have no issue with slaughtering those who resist them. Indeed, the Shedai word for peoples outside their hegemony often doubles as a synonym for "base criminals" or "uncivilized beings". Even the Apostate, who believes in benevolent rule and rejects the idea of conquest, seems to think the Shedai are natural leaders, above all other species.
In the Star Trek: Deep Space Nine Relaunch, the Kurlans are completely unapologetic about their infestation of humanoid beings, insisting that humanoids are simply "meat". Whenever someone tries to reason with one of their number, it responds only with sneering contempt, mockingly explaining that humanoids "think with their glands" and know nothing of true intelligence.
On the rare occasion that the Animorphs directly interact with yeerk controllers, the yeerks almost always pull out this argument. As far as they're concerned, all other species are to yeerks as cattle are to humans, so yeerks are fully justified in enslaving, Mind Raping, and killing human, hork-bajir, andalites, and so on. Of course, the yeerks the Animorphs can interact with are the ones who think this — the ones who don't agree don't leave the yeerk pools in the first place, except a few who only changed their minds later.
In William R. Forstchen's The Lost Regiment series, the 9-foot-tall Human Aliens roaming the planet Valennia have no other word for humans than "cattle". Their people are the "chosen ones", while humans are there to serve them and fill their cookpots. Even horses (brought from Earth thousands of years ago and bred to carry the huge aliens) have a higher status than "cattle", since they enable their nomadic lifestyle. Other animals like pigs and cows are considered to be "lesser cattle", but the preferred meat of choice is "cattleflesh" (i.e. human meat). The aliens impose a Medieval Stasis on the various human cultures that come over periodically through the Tunnels of Light and have themselves maintained their level of technology for thousands of years, maintaining their way of life. Any new weapons that show up with newly-arrived humans are quickly destroyed or buried (last time, they tangled with some pirates and many Tugar warriors were lost to firearms before the pirates were killed).
The title character in Odd John believes that mutants like himself are justified in doing whatever they feel like to humans, no matter how cruel, because they're smarter than humans. Disturbingly, the author seems to agree.
In the Stargate SG-1 episode 'Pretense', a Goa'uld justifies the taking of human hosts by claiming superiority to humanity and comparing the practice to the hunting and fishing practiced by humans.
When Daniel Jackson points out that nearly all Goa'uld technology has been stolen from other races, the Goa'uld merely shrugs and says it doesn't matter how it was acquired. The Goa'uld have the technology; the humans don't. It's as simple as that.
What's better is that both sides are trying to convince a third party represented by a Nox, a race of perfect pacifists that already considers itself to be superior to both humans and Goa'uld.
And where are the proceedings taking place? On the planet of Technical Pacifists who feel morally superior to both Earth humans and the Goa'uld.
The aliens in V don't really think of themselves as a superior race, but consider the humans they covertly conquered as a resource to be consumed. At one point, the original miniseries has aliens offhandedly discussing how it was inadvisable to sedate human captives before butchering them because the drug alters the taste of the flesh.
In the Dungeons & Dragons episode of Community, Britta tries to tell a Gnome waiter that he's just as good as they are. Abed (as dungeon-master) replies that according to the game rules, no, he's actually not, and the gang are justified in treating him however they want because of this trope.
The Gua in First Wave view themselves as superior to humans because of the ease with which they are able to infiltrate and manipulate the human society. They kill without remorse and are determined to, eventually, take Earth for themselves. One episode features a Gua surgeon who explain to Cade that he views his experiments on humans in the same light as humans experimenting on rats. The Gua believe they are superior because they have managed to throw off an invasion of their own planet by a hostile race and transformed from a race of peaceful philosophers into conquerors (they have already taken at least one other world). The name "Gua" literally means "power to overcome". Some of their experiments are aimed at determining if humans possess this quality and are disturbed to learn that one in 117 humans do.
One female Gua even boasts how easily it is for their infiltrators to "sleep [their] way to the top". In another episode, a Gua cult leader reveals that Gua mating is painful and not at all pleasurable. Since all Gua on Earth inhabit human/Gua hybrids (called husks), they find human sex pleasurable and lack any sexual taboos present in most human cultures (likely another reason for the Gua to feel superior).
The Observers from Fringe see their takeover of humanity as justified by their superior technology and intelligence. Windmark even thinks of humans as animals.
In Empire Of The Petal Throne, humans are the ones who did this when they invaded planet Tekumel. The native species (Ssu and Hluss) were clearly sapient and living together in peace, and had about the kind of technology that we have now in Real Life. Humans had developed far a more advanced starfaring civilization, so "obviously" the Tekumelani species were inferior. Humans had no problem allying peacefully with other advanced starfaring species, but they terraformed the hell out of Tekumel, rearranged its orbit and even gravity, and tried their best to genocide the "primitive" natives. It's even noted that most other starfaring races wouldn't have invaded at all.
The Eldar in Warhammer 40,000 have this attitude. They are ancient, wise and (most importantly) a Dying Race. Humanity is young and stupid and there are already a lot of them around (humanity may also count as a Dying Race, but if they are they're dying a lot slower). Therefore, the Eldar feel completely justified in scouring a human planet clean of life (or changing the present so that this will happen to humanity in the future) to save a single Eldar life. The Dark Eldar do this to extremes, though the fact that they think nothing of doing it to themselves if they can means they probably go out the other end into Always Chaotic Evil.
Gideon: The torture, the terror, the raiding, the killing, maiming, stealing. Everything. Why?
Asdrubael Vect: Why should I not? You are of no consequence. If you had not been captured by my servants and did not fall foul of some illness or mishap, you would still die within another twenty of your planet's short years. Why should I not use such a pointless creature for my amusement and sustenance? You are prey-species, nothing more.
In Baten Kaitos this thought is enforced by the government schools of Alfard ("The Empire of the flame"). Lyude, the one heroic character from the country, is revealed to have been homeschooled by a nanny.
The fal'Cie from Final Fantasy XIII are all over this trope like flies over jam. At one point, Lightning realizes that to them, humans are nothing but pets whom they keep for amusement and some housekeeping chores they don't care to do themselves. It is eventually revealed that humans and fal'Cie are related species in the sense that both were created by the same creator deity but fal'Cie were made infinitely stronger but without the capacity for free will, so when the creator has left the building, things went south for the humans as the fal'Cie hijacked that free will to turn them into weapons.
Baron Alexander from Amnesia: The Dark Descent justifies the meticulous, brutal torture of thousands of human beings, optimized for maximum terror and pain, with the fact that he's an immortal being from another dimension, and torture of sapient beings is the only way to gather vitae that allows him to work towards getting back home. To his dubious credit, there isn't a hint of sadism in his actions; all the torment is carefully calculated and purpose driven without any kind of emotional dimension. However, at one point when examining one of his memory things he admits that he's a monster who's done unforgivable things, but he just wants to get home too badly to stop.