Thou Shalt Not Kill Muggles

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. 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.

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.

Anime And Manga

 * 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.
 * The mages from Fairy Tail form guilds to take odd jobs, but it is illegal for a guild to accept an assassination mission. That said, 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.
 * And yet the Student Police are convinced it's okay to execute any human who discovers Youkai Academy. By the way, the school's supposed to teach coexistence.

Literature

 * Animorphs: the group never kills Yeerk-infested humans, but has no such restraint against Hork-Bajir or Taxxons. This later backfires on them when Visser One realizes only humans wouldn't kill humans, Andalites would have had no issue killing them.
 * In most Asimovian fiction, this rule is in full effect with "Three Laws"-Compliant robots; the first law is that they cannot harm humans. It does suffer from some Loophole Abuse, though.
 * 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.
 * 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 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.

Live Action Television

 * 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 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.
 * Season 4 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.

Video Games

 * If, in the Assassin's Creed series, you repeatedly kill civilians, your "Synchronization Meter" (Health Bar) goes down. And if you kill more than three civilians in a short frame of time you desynchronize. The reason given is because the protaganist (Ezio or Altair, usually) never killed anyone because it would go against the first tenet of the Creed.
 * This is adverted 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.

Real Life

 * Most developed countries frown heavily on a police or military force that causes unnecessary civilian casualties.
 * Doctors have knowledge of human anatomy that could be dangerous if misused, but they're under oath not to use their skills or position to harm people.
 * Martial arts instructors place heavy emphasis on discipline and on primarily using the skills they teach for defense.