Can't Default to Murder

Batman: No killing!
Catwoman: Where's the fun in that?

— The Dark Knight Rises

A character is more, er, let's say morally flexible than most. This character may be a Reformed Criminal, live by an entirely different set of rules, are used to solving every problem with force, or are just straight-up evil.

They find themselves in a reluctant alliance with a person or group who has a strong sense of right and wrong, believing that they should only use violence when absolutely necessary. Killing their enemies is a line they will never cross, it's an absolute non-starter.

Philosophies may clash, but the non-killer(s) are in charge, and their word is final. They demand that the Morally Gray character avoid violence whenever possible. And definitely no killing. The morally gray character reluctantly agrees but is unused to approaching obstacles in this new way. Shenanigans ensue.

The morally gray character might appear compliant for a time, but when given the chance, or when put into high-pressure situations, they usually revert to their old instincts and kill their enemies, much to the non-killer's chagrin.

This trope is useful for situations where Violence Really Is the Answer and the only realistic way to stop a villain is by killing them. By having the morally gray character disobey orders and kill the villain, the story preserves the innocence of the non-killers. It serves a similar function as Self-Disposing Villain.

In the end, the morally gray character may have a change of heart. But just as often, the two find their views totally incompatible and break off their partnership in some way.

Sub-Trope of Thou Shalt Not Kill and Teeth-Clenched Teamwork.

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Examples:

Anime & Manga

• Dragon Ball Super: One of the rules of the Tournament of Power is that the fighters aren't allowed to kill their opponent or they'll be disqualified. Frieza, who has zero problems killing anyone, is stuck having to restrain himself. However, while he can't kill anyone in the tournament itself, there isn't a rule that he can't make it so someone is at risk of dying after they're disqualified from the tournament.
• Happy Sugar Life: This is one thing that separates Satou Matsuzaka from most characters of the Yandere archetype. She has killed, but she mostly uses subterfuge and cunning to get rid of any threats to her life with Shio. There are only two instances when she resorted to it. The first was her first kill when she initially realized she loved Shio and was done on impulse, and it was to protect Shio from an attacker. The second is Shouko, and she had only done so since she was going to expose her, and felt betrayed by her claiming to be a friend. Most other times, she has to restrain herself from committing any violence.
• After the Time Skip and Changing of the Guard in Moriarty the Patriot, murder is off the table as an option to solve the Moriarty crew's problems. It takes them a bit to adjust to the new status quo.
• One story arc in Spy X Family has master assassin Yor hired for an unusual mission: instead of killing a target, she has to serve as a temporary bodyguard and ensure the safe passage of a woman and her baby fleeing several other assassins on a cruise ship. This doesn't stop her from killing enemies in the process of protecting them, though.

Comic Books

• Spider-Man: In The Amazing Spider-Man (1963) #161-162, Spider-Man was forced to team up with The Punisher, Spidey enforced his No-Killing rule by making Frank use rubber bullets. Frank complied, both because they didn't have any time for arguing and because this was very early in Frank's history before he became the Garth Ennis-molded Blood Knight he is now. Of course, a rubber bullet to the head or throat is just as lethal, and an experienced Marine like Frank could have swapped out magazines holding real bullets without Spidey ever noticing. Other heroes, such as Captain America or Daredevil, have also tried to make Frank refrain from killing when teaming up with him. He doesn't always comply.
• Suicide Squad: Conner Kent is forced onto the Squad by Amanda Waller. Though he is forced to go along with and help this team of violent psychopaths and bloodthirsty vigilantes, he steadfastly holds to his mentor's ideals by refusing to let his teammates kill anyone. This makes him the most despised member of the team, with Nocturna stating how much she hates him and Peacemaker punching him out while he's under the effects of kryptonite.

Films — Animation

• Superman vs. the Elite: The new superhero team known as The Elite arrives on the world stage and starts dispensing vigilante justice, but unlike Superman, they believe that they need to kill their enemies to make sure they can do no more harm. In their first meeting, Superman forcibly stops their leader, Manchester Black, from torturing two terrorists to death. Later, he fails to stop Black from killing a legion of terrorist soldiers, and later the supervillain Atomic Skull.

Films — Live-Action

• In Terminator 2: Judgment Day, the reformed Terminator is programmed to terminate threats to John Connor's life. John insists that he protect without killing. Hilariously, after the first time John says this, the Terminator incapacitates a guy by brutally shooting him in both legs, to John's horror. The Terminator justifies it by saying "he'll live", showing that while he may not kill people, the Terminator can still be brutally effective.
• Parodied in Deadpool (2016). Wade Wilson has the Mad Scientist who turned him into a mutant at gunpoint at the climax, and Colossus starts into a Patrick Stewart Speech trying to convince Deadpool to let the man live—and Wade shoots the villain in the head mid-speech and starts complaining about Colossus monologuing.
• In Deadpool 2, Deadpool tries to apply the maxims Colossus tried to teach him in the first movie, echoing many of the lines Colossus used (and Colossus is in the background smiling happily). Then of course, Wade realizes the people he's trying not to kill are child abusers, and kills them, in full view of the police.
• The Dark Knight Rises: On the occasions when Batman and Catwoman are forced to team up, Batman insists on not killing their opponents, which Catwoman does not agree with. At the end, after Batman has been captured by Bane, she shows up on the Batpod and blows him away, commenting, "About the whole no guns thing, not sure I feel as strongly about it as you do." Batman doesn't protest, possibly because Bane was about to kill him, but more likely because they have a nuke to disarm and they're on the clock.
• The Empire Strikes Back has Darth Vader hiring Bounty Hunters to find the Millennium Falcon, specifying that he wants the Rebels alive, and stresses to Boba Fett, "No disintegrations!" which implies Fett has previously disintegrated a target Vader or the Empire had wanted alive.
• Venom (2018): Eddie Brock is horrified when the Symbiote uses his body to kill and eat their enemies. Eventually, they come to a relatively stable truce, where Eddie insists that they "only eat the bad people."
• Mission: Impossible (1996): In the lead-up to the famous Langley vault heist with Ethan Hunt, one of the IMF outcasts that Ethan recruits is an assassin, Franz Kreiger. One part of their plan involves no kills or grievous injury to their opposition. When the group infiltrates the Langley CIA complex as firefighters, Hunt and Kreiger dispose of their suits in a janitor's closet. However, they are accosted by a guard as they do so. Kreiger disables him and suddenly unsheathes a hunting knife. It takes Hunt's quick realization and reflexes to keep him from either slitting the guard's throat, backstabbing him, or worse, eviscerating him.
  [Ethan pins Kreiger's knife-clutching arm to the wall]
  Ethan Hunt: Zero body count.

Literature

• Journey to the West: Sun Wukong frequently has to be held back from killing people by the Buddhist monk Xuanzang via the enchanted headband stuck on his head, even when it's genuinely the best solution.

Live-Action TV

• In Timeless Season 2, Lucy teams up with Flynn, the antagonist from Season 1. On their first few missions together, Flynn suggests murder in response to an obstacle several times, which Lucy always shoots down right away.
• Daredevil (2015): Despite having a no-kill policy himself, Matt Murdock has a habit of teaming up with killers (his mentor Stick, his old flame Elektra, and murderous vigilante Frank Castle) and doing his best to stop them from committing murder while working with them, even if it means knocking lethal weapons out of their hands mid-battle. Late in season 2, Matt solicits help from Frank Castle and actually suggests that "just this once" he'll do it Frank's way and kill their enemies. Frank immediately calls him out on compromising his principles, tells him that once he crosses that line "just this once" he can never go back, and refuses to work with him.
• Jessica Jones (2015): In season 2, Jessica is reunited with her mother. After a near-fatal car crash, IGH did an experimental procedure on her which saved her mother's life, but changed her personality. Now, she has super-strength and uncontrolled rage. While on the run from IGH, she kills several people despite Jessica's repeated insistence that she avoid killing.
• The Tick (2016): After The Tick saves Overkill's life, Overkill decides his life now belongs to The Tick, as The Tick being Nigh-Invulnerable means there's no way he can repay him in kind. Tick decides to cash in the debt by ordering Overkill to stop killing people, who is horrified and disgusted but still forced to agree. In a later episode, Dot points out that she too has saved Overkill's life and uses that life debt to erase the no-kill order.
  Overkill: I need lethal capability!
  Tick: You need... a hug.
• Doctor Who: After the warrior Leela starts travelling with the Doctor, the Doctor has to keep reminding her that he would prefer her to solve problems by means other than killing whoever is currently the problem.
• In the Star Trek: Voyager episode "The Void", Janeway forms an alliance with the crews of several other spaceships but makes three rules: no killing, no stealing, and no giving up. Most of them follow these rules fine, but one murderous alien breaks all three: he gives up on trying to follow them and then kills a man to steal a piece of equipment.
• Buffy the Vampire Slayer: in Season 4, the vicious vampire Spike is captured by the military and has an antiviolence chip implanted in his brain, leaving him unable to kill or even harm humans (and essentially leaving him a neurotic mess, since that's his favorite hobby). Subverted when he learns the chip only responds to humans, and that he can kill demons and other supernatural creatures at will. And he does. Repeatedly.
• Killing Eve: Subverted. In season 2, Villanelle is forced by British intelligence to agree to avoid killing anyone while she's working with Eve to take down Aaron Peel. She doesn't hold up her end of the bargain. However, British Intelligence was hoping from the beginning that she would disobey their orders.
• The Flash (2014): In Rogue Time, Leonard Snart a.k.a. Captain Cold has discovered The Flash's secret identity and is using it as leverage, threatening to expose him if he doesn't stay out of Snart's business. Barry asks if Snart thinks he'll allow innocent people to suffer to protect his secret. Snart is forced to admit that he won't, and so they come to a compromise: Barry won't arrest Snart as long as he keeps Barry's secret and doesn't kill anyone. It helps that Barry plays it like a dare, saying that a criminal mastermind of Snart's caliber could easily do his work without killing anyone, something that Snart admits and takes up as a challenge.

Video Games

• Just like in the movie, your Terminator in the NES version of Terminator 2: Judgment Day takes an oath not to kill anyone on John's orders. This means during stages 3 and 4 that you either have to avoid the human guards or only shoot them in the legs. Killing them incurs a points penalty, and if you lose too many points you won't be able to get the strongest weapons for the later stages of the game.
• Injustice 2: When the murderers in the Regime are forced to ally with Batman's team to fight the bigger threat, Brainiac, they are forced to operate under Batman's no-kill rule. This ends up causing problems later when Wonder Woman attempts to kill Cheetah and Harley Quinn attempts to stop her citing the no-killing rule.
• Danganronpa Another Episode: Ultra Despair Girls: In order for Toko, and by extension, her, to be able to join the Future Foundation, Genocide Jack is doing her best to not kill anyone while helping Komaru through Towa City.
• Alpha Protocol: Steven Heck is available for partnering up for both guidance and assistance, no matter how pacifistic Mike Thorton is. At a given point you can tell him casualties aren't allowed, which dings your relationship, saddens him, and in the end doesn't even work because he goes guns blazing anyways.
• Mass Effect: Paragon Shepard doesn't kill unarmed people who are surrendering and tries to prevent his/her squad members from doing so as well. This is especially prominent in Mass Effect 2, where Garrus, Miranda, Jack, and Mordin all wind up being prevented from killing someone who wronged them during their loyalty missions. Even Jack is willing to agree that Shepard is right about it.
• In Mortal Kombat vs. DC Universe, when The Joker, Lex Luthor, Catwoman, and Deathstroke are forced to align themselves with the DC superheroes against the Mortal Kombat universe, they are given orders to refrain from killing anyone, much to The Joker's dismay. However, when the Clown Prince gets infected by the Kombat Rage, the power goes to his head as he challenges Batman in Gotham and wins, only to get electrocuted by a taser from Bats as he celebrates his victory.

Webcomics

• The Order of the Stick: The Token Evil Teammate Belkar is at one point placed under a curse that inflicts a debilitating and potentially deadly affliction on him if he ever ends another life while within the bounds of a settlement. This forces him to abstain from his usual murderous ways for an arc or two until the curse gets lifted.
  • Later, while he and Haley are travelling with Cecilia, the former two frequently butt heads with the latter whenever they find themselves faced with a potential combat situation, as Cecilia is vehemently against the idea of taking others' lives, while Haley has no problem doing so if the need arises (and, as mentioned above, Belkar doesn't even need a reason). Sometimes, Cecilia is able to convince at least Haley to go along with a pacifistic means of bypassing such obstacles.
• In Schlock Mercenary, the titular Extremely Omnivorous Heroic Comedic Sociopath often has to be specifically ordered to not kill and eat anyone, though since the characters are mercenaries, and much of the time Violence Really Is the Answer, this is usually just temporary.

Western Animation

• The Fairly OddParents!: Principle Waxelplax hires truancy officer Shallowgrave to bring Timmy and Adam West to school once she finds out they're playing hooky, specifying they need to be brought back alive. Shallowgrave checks his Hook Hand collection and says he doesn't have a hook for "Alive," so he charges extra.

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