Normal humans exist in 99.9% of fictional universes. This is only natural, since Most Writers Are Human. But in some worlds, there are also races of humans with superior abilities, or perhaps powerful non-human races. So what prevents Social Darwinism from setting in? This trope.
Generally speaking, humans tend to end up in charge regardless. Whether it's because of technology, intelligence, social organization, sheer numbers, or some combination of the above, they become the dominant race, leaving the fantastic people with two options. The first is to form an Anti-Human Alliance. The second is to quietly coexist. However, due to Fantastic Racism, that doesn't always go smoothly, so for the fantastic races to reassure the Muggles that they can be trusted, strictly-enforced rules are often required.
For this trope to truly be in play, the rule against killing humans must be absolute and inflexible, and it is almost always one-sided, so more often than not, it gets deconstructed: for example, looking at this rule from the other (non-muggle) side of things, or abuse of the rules. This can draw parallels with discrimination in Real Life. Sometimes, it can even extend to inflicting any sort of bodily harm on muggles at all, even if it's self-defense. This can really suck if Humans Are Bastards. On the other hand, provisions for such circumstances are vulnerable to Loophole Abuse. Gray And Gray/Black-and-Gray Morality are often present in such cases.
Compare Alien Non-Interference Clause, which can often be very similar.
Note that for this trope to be in play, Thou Shalt Not Kill must be averted. In other words, fantastic beings are allowed to kill other fantastic beings, just as long as they don't hurt the muggles. This can carry Unfortunate Implications, but it's sometimes justified, particularly when the non-muggles are significantly more powerful than the muggles, to the point where any notion of needing to kill a muggle in self defense is absurd and thus all acts of killing muggles are nothing but senseless, inexcusable violence. Compare What Measure Is a Non-Human?.
- Horribly averted in Fullmetal Alchemist. Even though the popular opinion is that alchemy should be used for the good of the people, the corrupt government (specifically President Evil Bradley) ordered the State Alchemists to annihilate the nation of Ishval, even when they tried to surrender! It's partially for this reason that State Alchemists are shunned for being dogs of the government, so in a sense, this trope can be felt in the atmosphere.
- The eponymous characters of Claymore aren't allowed to kill humans, even if it's to protect other humans. Any Claymore who does so is sentenced to death. And yet Awakened Beings are allowed to do as they please. Then again Awakened Beings aren't really part of the organization anyway.
- One plot point from the 4th War Arc of Naruto is guarding the daimyo, the Muggle leaders, because since they are the ones ninja answer to, capturing them will let Tobi put pressure on the Allied forces to hand over the 8 Tails and 9 Tails.
- The mages from Fairy Tail form guilds to take odd jobs, but it is illegal for a guild to accept an assassination mission. Thus there are many illegal guilds that the government has such a hard time keeping down that they hardly even bother.
- According to the bus driver in Rosario + Vampire, monsters aren't allowed to harm humans for any reason, even to save their own. Fortunately, Tsukune is technically still considered human. Yet the Student Police are convinced it's okay to execute any human who discovers Yōkai Academy. By the way, the school's supposed to teach coexistence. When you consider that the leader of the Student Police, Kuyō, is a spy for the Fantastic Racism organization Fairy Tale, it makes a bit more sense.
- Blood-C: Due to a pact she made with an unknown person, Saya is literally incapable of harming humans for any reason. The Elder Bairns are fair game, however.
- In Yu Yu Hakusho, it's a serious crime for a demon to kill a human under any circumstances. It becomes a plot point during one arc that Yusuke and Kuwabara have to rescue Yukina from crime boss Tarukane before her brother Hiei kills him for kidnapping and torturing his sister.
- Jonathan Hickman's X-Men: One of the three laws of Krakoa is 'kill no man'. Killing mutants is fine because they can be resurrected.
- Rosario Vampire: Brightest Darkness: It's been stated that angels are forbidden to harm humans for any reason unless said humans die with sin; this also applies to monsters who were once human, such as Hokuto. Being forced to Mercy Kill an HDA soldier to spare him a more agonizing death at Ceal's hands causes a lot of guilt for Gabriel. Now that Rason is the guardian angel of the monster world, he is the only exception.
- In one book, it's revealed that the group has never killed a Yeerk-infested human, but has no such restraint against Hork-Bajir or Taxxons. (Yes, you should ignore the many, MANY times they killed human-Controllers before this.) This backfires, as Visser One takes this as evidence that at least some of the "Andalite bandits" are actually humans.
- In a somewhat related example: in Book #26, Crayak wants to genocide the Iskoort while the Ellimist wants to preserve them. As such, the Animorphs and Erek are sent to their planet to fight seven Howlers over the species' fate. Eventually they realize that the Howlers can't kill any Iskoort until the "game" is over, making crowds very convenient.
- In most Asimovian fiction, this rule is in full effect with Three Laws-Compliant robots; the first law is that they cannot harm humans, nor allow a human to come to harm. Many of his stories are about how this general rule fares in specific circumstances.
- Forcefully imprison a human as long as the human is not physically harmed.
- Lie to them to avoid hurting their feelings only for the lies to cause them greater emotional harm.
- Arrange for them to be fired from their job, so long as they can arrange for another job.
- One set of robots didn't have the second part "nor allow a human being to come to harm" and so they could arrange for an "accident" and then refrain from saving them from that "accident".
- In the Discworld novel Sourcery we're told that wizards don't kill non-magic users because: 1) They seldom notice them 2) It's not sporting, and 3) Who'd do the cleaning up and prepare meals and so on.
- The Dresden Files feature an interesting edge case for wizards: they're not allowed to take another human life specifically by magic because that would constitute a breach of the First Law of Magic, count as "black" magic that permanently taints their soul, and schedule them for a quick execution by the Wardens of the White Council the moment the latter find out about it. No such rule protects humans from any other supernaturals (although a Masquerade is in effect to avoid having humanity at large try to come after the supernatural world as a whole) nor the latter from any wizards they may clash with, however, and the Council is also quite happy to leave any cases of wizards killing muggles by nonmagical means to the mundane authorities and not concern itself with those either.
- In Harry Potter, there are a lot of laws in place to protect the muggle population from physical harm (though mindwiping them to uphold the Masquerade is government policy), which are frequently alluded to. The punishment for killing a Muggle is the same as that for killing a wizard. The Death Eaters throw them out the window the first chance they get.
- Shadowhunters and Downworlders in The Mortal Instruments are forbidden by the Law to hurt mundanes.
- In Twilight, the Cullens are vampires who adhere to a strict "vegetarian" diet which means they only drink the blood of animals and not humans. Unfortunately, this doesn't mean they're required or able to stop OTHER vampires from killing the humans they themselves won't eat.
- In Buffy the Vampire Slayer
- Buffy sometimes fights against "normal" humans, but usually tries not to kill them. In one instance, accidentally "killing" a human-looking robot in self-defense is enough to give her a Heroic BSoD.
- The concept is explored further when (other vampire slayer) Faith accidentally kills a human while in the middle of a vampire fight. Giles tries to explain that such accidents are, historically, not uncommon for Slayers. In the grand scheme of things, it's seen as a small price to pay for all the vampires, demons, and other nasties that a Slayer kills on a daily basis. The problem in that case is that she handles the situation not by accepting responsibility for it (even shielded from real legal consequences), but by Jumping Off the Slippery Slope.
- In Grimm, Grimms (or at least the ones we know like Nick and Trouble) try not to kill normal humans, it even became a plot point after Nick accidentally kills one during a supernaturally-induced rage. Renard even points out the fact that his guilt was a double standard.
- Lucifer (2016): Angels, even fallen ones like Lucifer, are forbidden by God from killing humans... directly. Loophole Abuse like loosing a demon on a human, blackmailing one human to kill another, or (as the Archangel Uriel loves to do) engineering Disaster Dominoes that result in human death is fair game.
- The Unknowns from Kamen Rider Agito are forbidden from killing normal humans (their MO is killing those with special powers). One unlucky Monster of the Week had this demonstrated on them the hard way.
- Kamen Rider Amazons uses this to demonstrate Jin's morality. At one point, Haruka is about to kick the stuffing out of a thoroughly unrepentant serial killer who had just beaten one of the NPS members half to death, only to be stopped by Jin, who continues to knock Haruka down as he tries to attack the serial killer and explains that, horrible as the man is, he's still human and Jin only kills Amazons.
- Season 4 of True Blood gives a partial example of this trope as the Vampires attempt to recover from Russel Edgington's stunt on live television. All the vampires are strictly forbidden from killing/horribly maiming humans... if there's a chance they can be caught on camera.
- In the Supernatural episode "Freaks and Geeks", Dean stops Krissy from killing Victor, The Man Behind the Monsters who killed her father, because he's entirely human and "we don't kill people". Despite the Winchester brothers' own extensive human body count and eagerness to kill human victims of Demonic Possession as long as they take the demons with them.
- In Mage: The Ascension this is a big deal for both the Traditions and the Technocracy, as both have strict rules about not letting "sleepers" become casualties of The Ascension War. Both factions may treat that rule as. . .flexible. . .at times, but it's still officially a major rule for both sides.
- Over in Mage: The Awakening, this is not such a major issue. The Seers view mortals as sheep to use and abuse until their utility runs out and have an entire Order dedicated to keeping the Fallen World in a continual state of low-level war, while the Diamond's general disdain for Sleepers occasionally tips over the edge into open hostility (for example, there's a Mysterium philosophy that views mortal society as harmful to magic and believes all mortal nations and religions must be shattered). The Free Council are the one major exception: as they believe humanity is innately magical and all human creations have ties to arcane secrets, they are way harsher on harming the Sleepers than even the rest of the Pentacle - they're more tolerant of Left-Handed magic if it's directed at the Seers, but while the Diamond Orders prefer not to execute other mages unless it's deemed a grim necessity, the Free Council will absolutely kill you if they deem you guilty of abusing Sleepers.
- Ars Magica's Order of Hermes swears not to "interfere with the affairs of mundanes and thereby bring ruin on [other mages]." In theory, that enforces this trope on pain of death. In practice, people tend to interpret it as allowing anything that doesn't bring the Order into disrepute. If muggles disappear or meet unfortunate accidents in ways that don't get traced back to the Order, then, well, that's just a pity.
- If, in the Assassin's Creed series, you repeatedly kill civilians, your "Synchronization Meter" (Health Bar) goes down. If you kill more than three civilians in a short frame of time you desynchronize. The reason given is because The Protagonist (Ezio or Altair, usually) never killed anyone because it would go against the first tenet of the Creed. This is averted after beating the game, letting you kill anyone you want, especially those annoying beggars, without any ill effect.
- The demons in the Reincarnation series aren't allowed to kill any humans other than Reincarnies, however much they may want to. They'll always mention to the player if a method being attempted risks harming innocents.