This becomes a problem in the Mermaid Melody Pichi Pichi Pitch anime with Sara, a mermaid villain that appears late in the first season. When the princesses try to use their Magic Music on her, it has absolutely no effect, since it's apparently hard-wired into their powers that they can't hurt other mermaids. This, however, doesn't prevent Sara from sending them into fits of agony with her own Villain Songonce it's revealed that she too is a princess. The rule doesn't seem to exist in the manga, and the only reason they can't hurt her is because she's just that strong.
Appleseed the movie: "Bioroids don't kill other bioroids!" This time they are morally superior to humans, because that's what they were designed to be from the beginning. As a downside they can't feel positive emotions as strongly as humans, either.
At one point in After War Gundam X, Tiffa has a psychic chat with some Newtype dolphins, and explains human cruelty to them, since "Dolphins never attack one another". This is utter bollocks, since dolphins are actually well-known in scientific circles for their habit of killing one another for absolutely no reason other than "For the Lulz." Then again, it might be because the anime was made way back in 1996.
In the manga of Chrono Crusade, it is mentioned that a Demon killing another Demon is an unforgivable crime which brands the killer as a Sinner. The law-abiding Demons, the Pursuers, are then obliged to hunt down and kill all Sinners. So it's more 'Ape Shall Never Kill Ape Unless The Ape To Be Killed Is An Outlaw'.
From the New World: A thorough Deconstruction of this concept. Humans do not kill humans - even the idea is repugnant and incomprehensible. Not only are they socially conditioned to have a very strong aversion to killing or violence, they were also biologically modified with a death feedback response, a "self-destruct mechanism" that kills them with their own PK power should they ever kill another member of their species. The non-human bakenezumi have no such aversion to killing other bakenezumi and even fight wars for territory and slaves (under human supervision, as a controlled outlet for their aggression). This seems to play into the human perception of the bakenezumi as sub-human and potentially dangerous, though it certainly doesn't stop humans from using the bakenezumi in order to kill those humans who show signs of throwing off the social conditioning or who otherwise pose a threat to their social order. Finally, it's revealed in the very last episode that the bakenezumi were originally humans who lacked PK. Without the ability to give them a death feedback response, the same scientists who created the death feedback response instead mutated them into creatures that would no longer be recognized as "human," thus allowing humans to control them through the threat of violence even while creating their ideal pacifist society.
In Tellos, Hawke at one point manages to get out of trouble by invoking this trope when caught cheating at a betting game. Since it's forbidden for two members of the elvenUlfen race to raise arms against each other, he pretends to want a fight and then happens to remember that this is forbidden.
Planet of the Apes: The Trope Namer, specifically Battle for the Planet of the Apes (though the phrase "Ape shall not kill ape" appears three movies earlier in Beneath the Planet of the Apes as an excuse to not shoot down protesting pacifist apes). As the Real Life section shows, this is far from fact among actual primates.
The reboot (specifically Dawn) has kept this trope however, a No True Scotsman invocation helps Caesar get around it. Given that said ape had already attempted to assassinate Caesar and killed other apes, the others don't object too much.
Several Aliens tear apart another to acidically burn though the ship. However, it apparently did this voluntarily, sacrificing itself to allow the others to escape. Even if it wasn't, it wasn't petty "murder" but a necessary step to allow their race to survive. That, and these particular aliens had been splices with human DNA, so they weren't like normal Xenomorphs...
Whether it's more human or alien, the Newborn at the end viciously averts this. It's first act after being born is matricide. Its second act is to crush a human soldier's head. The only person it doesn't try to hurt is Ripley.
In X-Men: First Class, Sebastian Shaw lightly scolds Emma, after she punts Erik off their yacht, that, "We don't hurt our own kind." Later, however, he kills Darwin, and his team later go all-out to hurt/kill the X-Men; and of course, he beats up Erik on the sub, while trying to convince him to change sides.
This is one of the original Seven Commandments of Animal Farm, conveniently discarded when Joseph StalinExpy Napoleon became convinced there were traitors in his midst.
In Poul Anderson's Dominic Flandry series (part of his Technic History), Flandry is an Agent for the Terran Empire. Kidnapped by an alien race, the alien race asserts that they are far more civilized than the Terran Empire, as they would never betray an oath or otherwise be dishonest (except to other, lesser, races, like humans). He soon has the entire leadership of the planet backstabbing each other, noting that their refusal to admit that they, too, can betray each other if the price is right, is what enabled him to succeed in destroying them.
In the Dragonriders of Pern novels, dragonriders consider killing one another to be unthinkable, because the death of a rider means the rider's dragon dies as well. When an Ax-Crazy Oldtimer attacks F'lar, he doesn't dare to fight back with lethal force until it's revealed that the man's dragon just died in a failed attempt to mate with a queen dragon. Once F'lar knows the situation, he kills without hesitation.
Ender’s Game features an odd kind of aversion when a major difference between the Formics and Humanity is discovered. The Formics are an insect-like hive species with millions of drones controlled by singular Hive Queens. They assumed humans functioned like that in their first encounters and simply disposed of what they presumed to be mere drones. The Formics didn't conceptually understand that anything could be "sentient" that wasn't the queen of a hive-mind. When they realized that each individual human was a single sentient creature, their guilt over the number of lives they had taken was enough that they essentially accepted their own near-extinction in retaliation.
Gullivers Travels concludes with Gulliver visiting the land of the Houyhnhnms, (a race of intelligent horses) who keep the Yahoos (a race of unintelligent humans) as pack animals, somewhat analogous to how people treat horses. The Houyhnhnms, however, insist that they're better than humans because of how humanely and reasonably they treat each other, whereas we're constantly making wars. They don't treat Yahoos badly — no worse than we treat horses, anyway — but they refuse to try to help human society achieve the level of harmony they claim to have. They don't really have anything to teach Gulliver except that Humans Are the Real Monsters and that Houyhnhnms are great. Considering how ridiculous he makes Gulliver's behaviour after learning this, it's pretty clear that Swift - for all that he was a bit of misanthrope himself - didn't think much of the Houyhnhnms either.
The Souls from The Hostcompletely neglect to realize that there might be something about humanity (or, hell, any species that isn't their own) worth saving, at least at first. The human version of the trope also makes a bit of an appearance, but given that the Souls not only stole our planet, but also our entire way of life, virtually all of our infrastructure, and the bodies of many loved ones, and are still somewhat set on capturing the rest of us and making us hosts as well... you can kind of see the logic in not particularly liking them much, no matter how "altruistic" and "nice" they may otherwise be.
The Paris vampires in Interview with the Vampire have only one rule: don't kill another vampire. Breaking that rule is punishable by death.
The Lord of the Rings: Non-villain example: The Hobbits are big on not killing each other. During the Scouring of the Shire sequence, Frodo's orders are not to kill the Hobbits working with the evil Men who have taken over the Shire. To paraphrase him, "We've never killed each other, and we're not going to start now."
However, the Miller Sandyman says that he heard that Frodo's mother pushed Frodo’s father into a river, and he pulled her in after him. So even if it’s only Malicious Slander, the idea of killing another hobbits is not new in the Shire. And Sméagol killed Déagol a lot of time ago, before Sméagol would be transformed into Gollum.
Technically Sméagol and Déagol are river folk; similar to Hobbits, but they are never said to be the same race. Also, they were not of the Shire, and Frodo's quote includes the distinction.
Particularly Anvilicious in The Once and Future King, which claims that ants, termites, and humans are the only animals that make war on each other. This is almost true (chimps also have wars), but just because other animals don't have war doesn't mean they never kill each other.
Averted in The Silmarillion. Elves killing other Elves is considered an unforgivable sin, but at three points the Sons of Fëanor kill other Elves in their pursuit of the Silmarils.
Sector General: Justified with the Cinrusskins: as a race of empaths, no sane Cinrusskin has ever killed another as the pain of death is shared with both the victim and the murderer.
Chains of Violence: In this Star Trek novel, there are the Tseesk, bird-like creatures who enslaved a human colony. They repeatedly talk about how their society cares for each of them, and how humans in the colony they found were nothing like this - and this somehow gives them the right to make humans into slaves and when humans revolt, the Tseesk declare that humans are too much of a threat to be allowed to exist free, and want to exterminate them. Then it turns out that once Tseesk occupied 14 planets, but then they started a Civil War which rendered 10 planets uninhabitable, one planet was inhabitable but all Tseesk there died (this was a planet humans had their colony on), another turned into ice world where surviving Tseesk degenerated into primitive tribes, and only one planet survived mostly intact, thoug they still have to rely on tech from before the war which they now cannot replicate. Not really friendly to each other, either.
Thieves' World: The purpose of the Blue Star Order is to produce very powerful Adepts to fight in the Apocalypse, so they strictly forbid the killing of fellow Adepts. So Adepts (who are not a particularly savory bunch) don't kill each other, because their kind are superior. This probably skirts a subversion, due to the reversal of causality.
In some stories focusing on the Adepts, there turns out to be a catch to this. One of the keystones to a Blue Star Adept's power is an individual secret that, if learned by another magician, allows the learner to claim the Adept's power (and if learned by a nonmagician can often render the Adept powerless anyway). A powerless Adept is useless in the final battle and is thus fair game. So Adepts never attack each other, nor do they conspire for anybody else to... but that doesn't mean there aren't ultimately lethal vendettas going on.
The trope is discussed shortly after the climax of Utopia. When forced to choose between saving the insane New Law robot Prospero and the virulently anti-New Law and anti-No Law Simcor Beddle, Caliban decides that Prospero's willingness to kill humans and Beddle's willingness to kill robots were equivalent acts of evil. But Prospero had destroyed other robots, even other New Law robots to achieve his goals, and Caliban had no evidence that Beddle had or was willing to kill other humans for gain. This made Beddle marginally less loathsome than Prospero, and so Caliban shot his friend and rescued his enemy. So basically, an ape that will not kill ape is a better ape than one that will, but that isn't enough to make the first ape a good ape in absolute terms.
Vladimir Vasilyev's Wolfish Nature duology takes place in an alternate world where humans evolved from dogs. One of the defining moments of canus sapiens sapiens history is the Bio-Correction in the 18th century, when the "wolf gene" was bred out of every person on Earth, essentially creating this trope. The mere thought of killing another human is repulsive. Doing it usually means the person goes insane after the act. Only trained agents (and then only if they go through proper mental conditioning) have a chance of coming out of a mission where they are forced to kill with their psyche relatively intact. Even then they still spend weeks or even months in psychological recovery. There have been no wars since the Bio-Correction. The plot of the duology involves the discovery of an enclave of "wolves" - non-corrected humans who can still kill. It's not so much that the whole world is afraid of them. It's the fact that any country with a live specimen can use that to clone a whole army of merciless killers. At the end of the second novel, the protagonist finds out from a geneticist that the Bio-Correction was a lie of the Clap Your Hands If You Believe variety - if people are conditioned to think they're incapable of killing and will go insane if they do, then this is what will happen. Artistic License - Biology here-Any decent geneticist understands that aggression is not a single gene that can be surgically removed.
Inverted in Fablehaven. If you aren't a dragon, killing one is considered an insult to their entire race and marks you for death. Luckily, most dragons are locked away in seven sanctuaries and only leave when The End of the World as We Know It is upon them, hence why Vanessa, Kendra, and Seth are dragon slayers and still alive at the end.
Zig-zagged in A Song of Ice and Fire. In the Iron Islands, it's forbidden for one Ironborn to shed the blood of another, but drowning them or disposing of them in a similarly bloodless manner is perfectly okay.
Live Action TV
In Star Trek: Deep Space Nine, the Founders' highest (or even only) law is "no Changeling has ever harmed another" (until Odo does it - and they ain't too happy about that, as you might imagine). However, as the founders and leaders of the Dominion, a classic example of The Empire, they've harmed pretty much everyone else.
The Founders themselves follow this law to the letter, but not always in spirit however. First in "The Die is Cast," the Changeling posing as Lovok did not intervene when Garak tortured Odo, but he did go back and help the two of them escape. Also in "The Adversary", the Changeling saboteur tries to convince Odo to escape with him, but is willing to leave him to die with the humanoids if he won't come. Then in "Broken Link", they afflicted Odo with an infection that would have killed him unless he returned to the Great Link for judgment. In this case at least, they admit to internal conflict over the decision (unheard of among the Founders otherwise).
The Ferengi follow this to a lesser extent: while murder of individuals and the like happens, Quark has a big speech to Sisko in which he points out that while Humans look down on the Ferengi for being greedy capitalists, the Ferengi themselves look down on the Humans and think they're "better". Ferengi never engaged in genocide, slavery, or atomic warfare, which Human history is full of. Indeed, the Ferengi have never even fought a large-scale interstellar war, instead peacefully resolving disputes by (ruthlessly) applying economic pressure, subjugating their own women, and selling weapons to other people who commit genocide and engage in atomic warfare.
Rule of Acquisition #17 - A contract is a contract is a contract... but only between Ferengi.
Battlestar Galactica: "Humans don't respect life the way we do," from D'Anna after the Cylons have exterminated billions of humans. Caprica-Six clubbing this self-same D'Anna over the head with a rock is later denounced as "the first act of Cylon-on-Cylon violence in our history" (though it's really not) during a discussion on executing human detainees. Hypocritically, a Five shoots Caprica-Six for speaking out against the executions.
They've more recently resorted to simply blowing each other to bits with their Basestars.
Moreover, Three's comment is incorrect, as Cavil-One killed all maturing copies of Daniel-Seven and then contaminated the genetic code out of jealousy.
The irony is that Caprica-Six prevented D'Anna-Three from committing another act of Cylon-on-Cylon violence. Patricide, in fact.
The Cylons justify their genocide of humanity with "they would have destroyed themselves anyway". We find that a whole tribe of Cylons actually destroyed themselves. Plus, the modern Cylons mostly destroyed themselves too.
Suffice to say, the Cylons were really big on hypocrisy.
Special Unit 2: A particularly nasty specimen of Gargoyle (a mass murderer of humans) makes this comment in the pilot episode.
The Terrians in Earth 2 are actually incapable of attacking their own kind. A bad guy takes advantage of this by wearing a necklace of Terrian bones, and until it's removed they can't kill him. Of course, when it does finally get taken off, he's dragged underground pretty quickly.
The Vampires of True Blood have nothing but contempt for humans and will kill them at leisure if they can get away with it. But kill another Vampire and you are in for a world of hurt.
There are cases of vampires killing other vampires, but smart ones do it in secret and clean up the mess afterwards. After Bill kills Longshadow (Eric's bartender), who was trying to kill Sookie (who had exposed his embezzlement of Eric's money), Eric (being the vampire sheriff of the county) is required to report the crime. When Bill points out that Eric would've killed Longshadow anyway for stealing from him, Eric agrees but claims he would've been more discreet than doing it in front of witnesses.
A somewhat interesting inversion in Dark Shadows. The Leviathan are instead forbidden from using lethal force against humans. Though it's more to avoid even larger problems. Or it's supposed to be, but Real Life Writes the Plot, and the storyline didn't fully make sense in that regard.
Averted in Kamen Rider Kiva, where the King and Queen of the Fangire are specifically tasked with killing Fangire who betray their race (King kills those who aid humanity, Queen kills those who fall in love with humans). Of course Maya, the Queen circa 1986, falls in love with a human, which is where our hero comes from. And then Mio, the 2008 Queen falls for the child of that previous union. Occasionally Fangire will fight amongst themselves for more petty reasons, like those attempting to kill Maya in order to gain her power, or the 1986 King's guards who attempt to prevent Maya from rescuing the imprisoned Otoya.
Routinely averted on Walking with Dinosaurs and its spinoffs (including Primeval). The very first episode has a pair of protomammalian cynodonts eating their own young, and it doesn't improve much from there.
On Babylon 5, "Minbari do not kill Minbari." Although this tends to be interpreted rather loosely at times or with certain qualifications, it was ultimately what saved humanity from destruction during the Earth-Mimbari War, when it was discovered that humans and Mimbari share souls, which effectively meant humans were Mimbari and thus shouldn't be killed.
Midway through the fourth season, this one goes right out the window after some slippery-slope logic leads to the Religious Caste population of a city being exiled into the winter wilderness. Most don't survive the long trip on foot to the nearest town. Not long after, the Minbari Federation falls into a full-on civil war.
Like True Blood, Buffy the Vampire Slayer carries this aspect, but with all demons. In season 4, after being implanted with a chip that shocks him whenever he attempts to attack humans, Spike (a vampire) becomes gleeful when he discovers that he still has the capacity to harm other demons and vampires. He's then ostracized by those who show contempt for anyone willing to kill "their kind."
Although given all the demon-on-demon violence we see throughout the series, and even more mentioned in backstories, it appears that the real problem Sunnydale's demon community has with Spike is that he keeps helping the Slayer.
Young Dracula's vampires have no reservations about killing humans, but killing other vampires is strictly forbidden.
In Lost Girl Fae apparently aren't supposed to feed on each other, as that's what humans are for. Despite this Bo, a succubus, regularly feeds on other Fae and no one objects. This may be because succubi (and the like) have a very pleasurable form of feeding so people are willing to allow it. It could also be that while feeding on other Fae is allowed, killing them to feed is illegal. That again is what humans are for.
The Observers in Fringe. In the fifth season, they subjugate our world and pollute the air to make it more breathable for them, consequently shortening human lifespans. When Windmark finds that Nina has been experimenting on Observers, he is disgusted and declares humans to be "animals".
In a sidebar titled "Ape Shall Not Kill Ape" in the Feng Shui supplement "Seal of the Wheel," it is noted that the Ascended do not look kindly upon members of the Lodge offing each other because they aren't precisely numerous and the loss of even one of their number is a weighty matter, and thus do not suffer those who make a habit of this to live. Only the Unspoken Name, the leader of the Ascended, can issue a sanction order to kill another Ascended.
The Eldar in Warhammer 40,000. Killing a fellow Eldar is an unspeakable crime (and not just because they're a Dying Race), which helps fuel that whole Moral Myopia about how a single Eldar life is worth the combined rest of all other sentient life in the galaxy because all the 'lesser' species do horrible stuff to each other. Depending on which accounts you listen to, the Tau may also be an example of this trope to a lesser degree.
During the Horus Heresy series it is made pretty clear that prior to the Isstvan V Massacre it was considered taboo for Astartes to kill another Astartes, verging on the unthinkable. It is worth noting, however, the original purpose of the Space Wolves, besides taking part in the Great Crusade, was to be the "Emperor's Executioners". As in, killing other space marines. Although the issue has been deliberately kept nebulous, it seems, given the hints in the literature, they may have performed this specific duty at least once before the Burning of Prospero.
Werewolf: The Forsaken: Not only does the Oath of the Moon say "The People Shall Not Murder the People," but it's a sin against Harmony to kill another werewolf. Which is tricky, as the titular Forsaken are at war with their fanatical cousins, the Pure, who a) outnumber them and b) don't give two shits about that little provision.
The shadow fey of the Ravenloft setting have the Law of Arak, which absolutely forbids them from killing one another. This, of course, doesn't stop them from harming one another in non-fatal ways, or from killing and abusing as many non-fey as they want.
In Magic: The Gathering, Black has many spells that can instantly destroy creatures, but only if they aren't black themselves (or are artifacts). Considering Black's domain is death magic, it makes sense that Black spells doesn't work on creatures that aren't living to begin with. This represents that the situation is in practice rather than idealism as the trope requires, conforming to Black's worldview. By contrast, White features a handful of spells that make a more arbitrary division in not affecting white cards, representing the spirit of the trope much more effectively.
The giff are a race of mercenaries who originated in the Spelljammer campaign but who occasionally appear in other Dungeons & Dragons settings. For the right price, they'll work for nearly anyone as soldiers, enforcers, bodyguards, thugs, legbreakers, or basically anything that involves fighting, but they will not accept a job that involves fighting other giff. Period.
The Nathrezim (Dreadlords) in Warcraft, who, out of all the Always Chaotic Evil demons have proven time and time again to be the cruelest, darkest and most corrupt of individuals, are forbidden to kill each other. In fact, disregarding this was the ultimate loyalty test Sylvanas prepared for Varimathras, and even then, it turns out he was faking the kill, as his victim survived and is still in league with him.
The Protoss of StarCraft aren't supposed to kill each other, for fear of falling into racial madness. So naturally they engaged in at least three civil wars since the racial madness and didn't go mad. Turns out they were just scared and created propaganda.
Similarly, in his inauguration speech, Emperor Arcturus Mengsk of the Terran Dominion says "From this day forward, let no human make war on any other human." That doesn't end up happening, mainly because the United Earth Directorate sent a taskforce to conquer the sector.
The video ironically subverts Mengst's speech, perhaps intentionally. Even as he declares that no human should make war on any other human, we see one battle-cruiser destroy another, presumably Mengsk's forces spreading his reach in the sector.
Averted in Mass Effect — members of one race will easily take aim against their kinsman. Shepard at one point can point out that since Garrus is a turian, he shouldn't want to harm Saren (also a turian), but Garrus explains that race is irrelevant with respects to dishing out punishment and if anything, he's even more eager to take out Saren because he considers him to be a disgrace to his species.
Some party members (including Garrus) will ask why Wrex is willing to fight other krogan. There's a little more justification here, as the krogan are a Dying Race at that point. Wrex brushes it off.
Wrex actually laments that his fellow Krogan are far more interested in fighting and dying in battle than trying to save their race. Wrex mentions that his progressive ideas to try to rebuild (i.e. stop killing each and focusing repopulating) was what got him exiled, or at least banned from becoming a Krogan leader. If he survives Virmire, he manages to put his plans to save his people into action.
Averted in Sword of the Stars, where it is revealed that Hiver clans routinely fight inter-clan wars that cause enough deaths to exterminate the human race several times over, and that the Tarka have turned political backstabbing and civil war nearly into an art form by being so accustomed to it. The Liir play this straighter but actually practice what they preach against other species as well provided they don't cause trouble; getting a Liir angry at you isnot a good thing. Subverted when it's revealed in the sequel that the Suul'ka, who the Liir always make an exception for, are actually very old Liir elders. The Morrigi really don't care what lesser species do as long as they're willing to trade and don't defile old Morrigi colony sites.
Fallout: Although the Death Claws, America's genetically-engineered leftovers from the all-destructive great war, are extremely aggressive creatures little better than animals that attack humans on sight, it's implied (from the modified talking Death Claws in the second series) that their basic pack-based society has a rigidly hierarchical, peaceful, ethical pack-based basic society. They were extremely loyal to the pack as a whole, treating it as a family unit rather than having individual families. Fights within a pack are unheard of, and the pack's leader controls many aspects of life, such as choosing and matching female and male deathclaws for reproduction.
During the Lonesome Road DLC for Fallout: New Vegas, Ulysses reveals that Caesar has instructed his Frumentarii never to kill any couriers since many of them are in fact working for his Legion, thus courier does not kill courier. Courier Six, the player character, can choose not to follow this creed however.
Gets deconstructed all the way in Arcanum, whose elves claim that they do not kill another elves because if an elf dies "unprepared", his\her soul will never be able to Ascend to a Higher Plane of Existence. (Tarant's elven thugs aren't so polite, though.) Naturally, when Wrath, a dweller of Hidden Elf Village is killed by poison, the first and only suspect is his dwarven apprentice (who also was stupid enough to sign a life-long contract with the elf). It's up to you to prove that Wrath was killed by an elf Sharpe for a woman they almost fought for and whom Sharpe now lives with as husband and wife (almost a perversion in "free-love" elven society) - or dig it even further and discover that "perverted" family idea actually came from Wrath, that Sharpe actually never killed him and that the bastard has committed suicide solely to frame Sharpe.
The Lunarians in Touhou make a pretty big deal about how disgusting it is that the creatures living on Earth have to kill each other to survive. Of course, that won't stop them from killing anyone on Earth who threatens their own survival on the "deathless" moon.
Hyenas from Digger are a tribe of hunter gatherers who view non-hyenas as Prey instead of People. If a Hyena is killed hunting prey, the dead's honor is not a major issue. But if Hyena Kills Hyena, then vengeance must be taken, and the status of the original victim is dependent on the status of the one who takes revenge. This comes into play in the plot when Digger, a wombat, kills a person who killed a hyena, whereupon the killer is revealed to have been another hyena. A friend of the victim tries to keep her friend's status intact by getting Digger officially inducted into the hyena tribe and making the wombat a Person.
In Sluggy Freelance, Aylee comes up with this argument in the midst of a loyalty tug-of-war between her friends and her race. Thankfully she came to her senses after seeing how readily her race will sacrifice each other for a meal.
In Dreamwalk Journal killing non-sentient species is justified as long as it's in self-defence. When it comes to sentients, you can rob, cheat and fuck them blind, but causing injury or death is unthinkable.
Very much averted with practically every known species of animal. Social animals such as wolves, lions, and chimpanzees are particularly known for killing members of their own species. Many species of rodent commit mass infanticide and cannibalism within their own families. Even plants compete with their own species for sunlight and water.
Touch a baby hamster and its mother will eat it. Sick and twisted Mama Bear, or what?
That actually doesn't happen as often as "common knowledge" would have you believe ("touch it and mom won't take it back" is overall quite rare among animals) - so long as the mother doesn't feel especially stressed, you can usually handle her babies for brief periods of time without fear of her harming them afterwards (she will, in most cases, continue to care for them). Mama hamster's actually more likely to eat her babies if their father is still hanging around.
You think dolphins are cute? You think they're the smartest animal, possibly morally superior to humans? Y'know that adult dolphins will sometimes bludgeon baby dolphins (sometimes their own offspring) to death, right? They also kill porpoises for no obvious reason. It's worth noting that this isn't particularly common behaviour, however, so these could just be the occasional dolphin Serial Killer. Still, rethinking that little smile of theirs?
Also dolphins can be pretty mean by human standards. Dolphin rape isn't uncommon and there's at least one observed instance of a group of dolphin males killing the offspring of a dolphin female because she wouldn't submit.
Among non-mammals there's the praying mantis and black widow spider. The latter's habits are right there in its name. As it turns out, praying mantis females only eat the heads of their mates if they're hungry or under lots of stress, and most black widow species are totally cool with letting the male go unharmed after mating (sometimes even letting them live on the web for a while!), but that's still something that one doesn't do in polite society. Mantises in general don't have any problem eating other mantises they can overpower.
Taken to the logical extreme with Tiger Sharks, who devour their own siblings in utero.
It is commonly known that ants keep slaves. After a war (which ant colonies have all the time), the victors will kill the losing queen and enslave all of the survivors from the losing side. Not just absorb them into the worker ant workforce, either, but coerce them (under threat of violence) to perform the especially menial stuff. There is at least one species who do not have a worker ant workforce of their own, living off of exclusively enslaving other ants.
Chimps. Dear God, chimps. A male chimp will kill a baby chimp (as long as he's sure it's not his, so it's not a total aversion of this trope) just to put the mother back in heat. The really effed-up part is that she will be a-okay with that and gladly have sex with the killer of her children. (Bonobos, which have sex with pretty much everyone as a means of resolving social disputes, are much less violent.) And if you think that's bad, Jane Goodall has recorded several instances of infanticidal cannibalism. Ape doesn't just kill ape, ape eats ape.
There are some prides with multiple males, usually brothers who worked together to drive off the previous male. They will still kill the previous male's cubs, kick out their sons, and mate with their daughters though.
Lions are one of the first animals mentioned when one talks of horny infanticidal fathers, and it's true when they get to replace the male(s) of the pride. But at least they wouldn't kill their own cubs, and the male plays a small role in raising them (babysitting while the lionesses are hunting). They're pretty much fathers of the year compared to other polygamous species like, say, bears and skunks.
While chimpanzees do kill and maim each other with surprising regularity, gorillas and bonobos do not. Gorilla males put on very aggressive displays when they think another gorilla isn't being properly subservient, but rarely attack with serious force. Bonobos usually settle their differences with loving.
When birds sing, it has two purposes: to attract mates, and to warn same-gendered birds that a given location is claimed territory. If a male bird infringes on another one's territory and doesn't leave quickly, they will fight to the death.
Rabbits are not above consuming another rabbit, especially their own young.
Plenty of species of snakes will kill and eat members of the same species of them. Sometimes they'll only do it if they're starving, or competing for a limited resource. Other times, they just do.
The Snapping Turtle is a really creepy aversion. Its biting power (twice that of a Great White) really isn't necessary for trapping and eating the small fish it usually eats. It is however very useful for eating other hard shelled animals — namely other turtles including Snapping Turtles. This is a species that evolved to be a more efficient cannibal.
Alfalfa is harmful to alfalfa of the same subspecies.
An extraordinary number of plants avert this trope, in a phenomenon called allelopathy, where they intentionally release toxic chemicals to inhibit the growth or kill other plants. This is especially common in seedlings, where they poison the surrounding soil to kill off their sibling seedlings to ensure that they have space to grow. Show that family love! Adult trees such as pines get on the action by covering the ground around them with thick needles, effectively smothering any seedlings or groundcover that dares to invade its personal space.
In hard times, wolf packs will commit cannibalism, overwhelming runts and tearing them apart. This is usually a desperate move, but quite jarring by human standards as wolf packs are family units. Similarly, it was discovered that in places untouched by humans, the most common cause of death for a wolf was being killed by another wolf, usually in disputes over territory. Like most animals, mother wolves will quite readily eat or abandon their cubs if they feel that there is no possibility of them surviving.
And, of course, plenty of human administrations, both religious and secular, have committed all kinds of atrocities against others, based on whatever criteria. Nazis didn't even treat each other all that well. Chronic Backstabbing Disorder was practically written into the ideology from the get-go.
Eugenics and other schools of thought that place one part of humanity above another give rise to the idea that "lower humans" aren't really humans at all. It follows that since they are not your kind you are free to treat them like animals. Those Wacky Nazis didn't invent that kind of philosophy; indeed, they probably did more than anyone to make it unpopular (or even unthinkable) through their example. But it's Older Than Dirt; in order to be enslaved a human has to be less of a man than his master, downgraded from a person to a piece of property like a horse or a dog, and slavery is as old as the first civilizations.
Not that the majority of slavery hasn't been essentially incidental, without any particularly heavy ideology behind it—just, this is the person who lost at war, this is the person who ran out of money, this is the person whose father sold her...and freed slaves quite often went on to own their own.
More recently, this is also what the Duro v. Reina decision opened up.
Inverted with the Betta fish. They're incredibly aggressive to their own kind, at times attacking their own reflection. However, for the most part, they leave other species of fish alone. Many otherwise peaceful saltwater aquarium fish cannot be kept with members of their own species due to intra-species aggression. Many other fish, especially aggressive schoolers, play this trope straight. Cichlids also will usually kill most non-cichlid fish as well, but can be kept with other cichlids, not because cichlids won't kill one other but they will (usually) reach a standstill.
The entire premise is inverted with the common aphorism that "The measure of a man is not how he treats his equals, but how he treats his inferiors."
Truth in Television for humans. The aversion that most sane, moral humans have against killing another human being in cold blood is very powerful, to the point that when we do it we usually find justifications for it, if only to keep from going insane.
Keep in mind it's more of a spectrum (similar to the graph the Uncanny Valley is placed on) the less relatable, the more justified. Dogs are a no kill (except in the Philippines), cows are justified for food, and insects are killed casually-by the thousands.
This is also subject to Values Dissonance — some cultures are okay with eating dogs, cats, and horses, and others consider killing cows to be taboo.
Most cultures historically also had little problem with enslaving, killing, or raping other people-so long as they're outsiders. Within the group it was (usually) discouraged.
Basically, this trope's existence could be chalked up to Most Writers Are Human, and humans are the only creatures known thus far who can kill their own kind and actually feel bad about it.
In general humans have a hard time actually considering killing someone else. To do so requires either an extreme emotional state or a sense of detachment, i.e. identifying the target as something other than one of us. We're also able to identify other species as "us", most notably dogs, cats and horses.
Actually, modern humans don't kill each other casually because we have the Social Contract Theory drilled into our brains since birth. Under this contract, Human A will not kill Human B, and in return, Human B will work together with Human A to protect each other from other humans. Since two or more humans cooperating will be much more likely to survive disasters or predators compared to a lone human, the humans on Earth nowadays consists completely of humans who subscribe to this contract. So, to be exact, it is Ape Shall Never Kill Ape Unless Ape Wants To Die.
Cats will almost never fight to the death, or even fight each other all that seriously; catfights are mostly noise and posturing to assert dominance, with claws rarely being used except for the occasional swipe to the snout. This is a survival trait: those razor-sharp claws and proportionately long, sharp teeth mean they could do very severe damage to each other. Which in turn means that in an all-out fight, even the "winner" would almost always be seriously if not fatally wounded.
A good analogy would be a human knife fight. There isn't really a winner, one goes to the hospital and the other goes to the morgue.
Royalty, generally, kings and queens will do their uttermost to not be seen participating in or condoning regicide in case the rabble gets ideas. A captured king can be deposed, ransomed, exiled, or have an "accident", everything except signing a death warrant. Thus Elizabeth I of England's reluctance to have her imprisoned cousin Mary, Queen of Scots put to death, even when a serious threat to her rule (Elizabeth was viewed as illegitimate by many, thus making Mary next in line to her throne). However, she eventually did it anyway. Mary's son James would succeed Elizabeth, ironically, and his son Charles I was himself beheaded after losing the English Civil War. Years later, the royal houses of Europe attacked France after they guillotined their king Louis XVI.
Sharks, at least Great Whites, will prefer to ram each other as a show of dominance and "get out of my space" because biting will possibly result in a Pyrrhic Victory. The accounts of divers that have been rammed rather than bitten by sharks is because they were mistaken for another predator and thus a threat rather than food.