Ending a life is usually a permanent thing. There's no way to say "I'm sorry", or to make up for it later. And for some people, killing is a line they will not cross, no matter how much the death might serve the greater good (or, in some cases, the greater evil). "He needed killing" is not in these people's vocabulary.
This is common in works with Black and White Morality, but even appears in works with Grey and Gray Morality. In the latter, it's sometimes the only way to tell the "good" guys from the "bad" guys.
Opinions differ on how this applies to sentient life other than humans. In general, it's still up to Big Damn Villains to kill other villains. It's perfectly fine to "kill" immortals though as it is to kill the undead. The Mercy Kill sometimes winds up as an exception. Karmic Death, Self-Disposing Villain, and Hoist by His Own Petard provide alternate ways to kill off villains without forcing the hero to get his hands dirty.
Thou Shalt Not Kill is closely related to Joker Immunity. Whilst many writers believe a never-kill creed makes the hero more likable and righteous, on another level it might simply be a plot device to prevent the hero from killing off popular recurring villains. This trope is more common in serial fiction, such as TV shows and comic books, rather than one-shots like movies. In action movies it is common and acceptable for the hero to kill the villain because there is usually no planned sequel for the villain to appear in.
Whatever the moral case is, this trope is often used to show off the hero's incredible precision, whether it be with a fist or a gun. This can include things like separating the mook from their weapon with a precisely aimed bullet, or possibly knocking an opponent out. Whatever the case, their non-lethal attacks are due to their incredible skill. Note that this often a case of Reality Is Unrealistic as many of these attacks are very capable of causing serious injury or death.
This is actually Truth in Television in quite an extreme manner. In general, as an evolutionary-based concept, there are very few humans who can even STAND the thought of TRULY ending the life of another human (without military training, of course.) In fact, the military has developed many studies over the years to evolve the best training curriculum to dehumanize the "enemy" and enable their soldiers to kill enemy soldiers. And even then, when a soldier is tasked with actually shooting a bullet that would end the life of another human being...many would rather intentionally miss. It is this (strength, in the case of all human life, but weakness in the case of human life with an opposing ideology) that makes modern militaries attempt to make the enemy soldiers appear as "unhuman" as possible.
See also Kick Them While They Are Down, Actual Pacifist, Reckless Pacifist, Technical Pacifist, Martial Pacifist, Non-Lethal Warfare, Would Not Shoot a Good Guy.
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Anime and Manga
In Dragon Ball Z, the main character Goku strongly personifies this trope (so much so that is almost considered a Running Gag) as he refuses to kill even the most evil opponents. He lets Piccolo Jr., a character who was at that time considered a demon who also just tried to kill him live, he tries to let his brother to live after threatening his son and rest of the population of the planet, and he even tries to let Frieza, a genocidal alien who destroys planets for fun and just killed his life long best friend, live.
It is odd because he was willing to kill an entire mook army as a kid and eventually is willing to kill evil opponents later in the series, but for a long period of time in the middle he insist on giving everyone a second chance.
In Frieza's case, Goku seemed to think letting him live as Half the Man He Used to Be was a greater punishment than simply killing him. When he realized that Frieza had been restored to full strength as a cyborg, he was going to finish the job. Trunks beat him to it.
And even then, at the finale of battle Goku clearly blasts Freeza at full power with intent to kill, but had to flee to try to save himself rather than confirm his death. Freeza just happened to have survived.
Other members of the crew, namely Zoro and Robin, are willing to kill off their foes. They never actually do end up killing anyone except some incredibly marginal characters, but they're definitely not holding anything back.
4Kids forces this on their One Piece dub. One example was how Lucky Roo shoots a bandit in the head, but the dub has Shanks saying that "And when he wakes up, tell him it's a cap gun!"
Fishman pirate Fisher Tiger held this as a strict rule among his Pirates of the Sun. It wasn't out of altruism. Instead he felt Humans Are Bastards and didn't want his crew falling to that level. He also hoped to avert encouraging a Cycle of Revenge.
Vash's quest to live without killing is essentially the main subject of Trigun. Not only does he strive to live without killing, he also attempts to spread this philosophy to others, including villains, even at the cost of his own health and safety. Other times, he begs characters who have a just cause for vengeance to forgo it and let things lie. However, he IS forced to take a life at one point to save others. This does not cause him to renounce his goal to save as many people as possible however.
Mazinger Z: The original manga plays with it. In one chapter, three Iron Masks sneak in Kouji's home to try murdering him (it must be stated in the manga they were WAY more competent than in the anime, where Law of Conservation of Ninjutsu held true). Kouji hesitates about killing them even after finding out they are corpses reanimated with a mechanical brain. When he finally gets forced to kill one of them in self-defense he suffers a Heroic BSOD (he remains kneeled, shaking and trembling), and later he is wondering if he is a murderer now. However another character reassures him it was self-defense.
Rurouni Kenshin has a similar plot, where the main character (a former assassin) has sworn to never kill again, and uses a specially designed sword that faces the wrong way, so opponents won't get cut by it, normally. Get him to switch the sides of the sword and/or unleash his Superpowered Evil Side, and you may have a problem.
In Claymore, the Claymores are forbidden from killing humans, even if it's an accident or is done to protect another human. The punishment is immediate execution. However, there have been Claymores who are perfectly willing to kill humans. For example, Teresa slaughtered a group of bandits to protect Clare then went rogue and there was Ophelia who took sadistic pleasure in torturing and killing humans and fellow Claymores, but was just sure to kill all the witnesses so no one would find out.
In Detective Conan / Case Closed, Conan/Shinichi always refuse to let a suspect end up dead, even the suspect tries to kill him. He would even try to save the suspect's life even if it risks his own life.
Nanoha Takamachi of Magical Girl Lyrical Nanoha. Very skilled at using Magical Damage, which lets her create a lot of flashy explosions without ever killing anyone, even sentient non-humans. By contrast, the Wolkenritter have no personal qualms about using lethal force when needed, but they also followed this code when they were antagonists in the second season since they didn't want Hayate's name to be defiled with blood.
One has to also understand that in a world where Defeat Means Friendship is the 45th law of physics, exterminating one's enemies permanently can be downright wasteful.
The Rave Master and his allies hold to this as a central philosophy. While they don't shy away from doing it when it's necessary, they will generally do all in their power to resolve their battles without killing.
In Bleach, most of the main cast members who aren't dead at least try to follow this, save for when fighting run-of-the-mill Hollows. Inoue Orihime not only hasn't harmed anyone seriously since her powers first emerged, but can't due to her personality. Unfortunately, she tends to believe that this makes her a burden on her friends, despite that she can reject reality to the point where she can heal a corpse with half of their brains blown out. The fact that it was someone who had shown up simply to try and torture her for no reason other than spite is just icing on the cake.
Oddly enough, Ichigo didn't kill a single Arrancar in his invasion of Hueco Mundo, at least not consciously; he let Grimmjow live, his Superpowered Evil Side took out Ulquiorra, and the mooks he cut down were revealed to be fakes generated by another arrancar. This is somewhat jarring when compared with Fake Karakura Town, where both of the Espada's Token Good Teammates are cut down without anybody batting an eye.
It's debateable whether Ichigo avoids killing because he's following a code, or because he just doesn't feel like it. In one arc the villain is revealed to have altered the past of Ichigo's friends and family, making them believe him to be their friend and turning on Ichigo when he attacks him. Once Ichigo hears the details, the first thing he asks if if they'll change back if he kills the person who did it. He looks quite scary when he says it.
Ichigo himself has issues with this. He has no will to kill anyone, and it's even lampshaded by Urahara. During his fight with Renji, he gets over it. Renji still lives, but Ichigo was able to put the thoughts behind him and kill him if necessary.
YuYu Hakusho plays with this trope when Yusuke fights Doctor Kamiya, a follower of Shinobu Sensui. Kamiya brings up that Yusuke has never killed humans before, only demons, and thus believes that Yusuke is going to hesitate before trying to kill him. He doesn't, and is only stopped when Kamiya takes a passing nurse as a hostage. Turns out the "nurse" is really Yana in disguise, and Murota informing Yusuke that the Doc was lying about a cure and really planned to finish him off, prompts Yusuke to cross the line. Killing Kamiya breaks his psychic territory and saves everyone in the hospital, and Yusuke's conscience is eased when Genkai quickly revives Kamiya with a chest compression.
In Digimon Adventure 02, the Digidestined are afraid of killing Digimon, and avoid it whenever possible (unlike in the first one, when fighting the forces of Etemon, Myotismon, and the Dark Masters). The only exceptions are Control Spire Digimon (as they are not considered "alive"), Kimeramon (who was created by the Digimon Emperor), MaloMyotismon (as he is pure evil and the main villain behind everything in Season 2), and Airdramon (for unknown reasons). They are forced to kill SkullSatamon, LadyDevimon, and MarineDevimon, and Yolei and Cody are shocked when this happens (though Cody knew they'd have to destroy MarineDevimon), though TK and Kari reassure them it was the right thing to do.
Fullmetal Alchemist: Main character Edward Elric refuses to kill to achieve his goals, even Homunculi. Still doesn't make him any less Badass by any stretch of the imagination. Just look what he did to Pride. He does ultimately end up (technically) killing the Big Bad, Father and accidentally kills one of his early opponents and later kills two homunculi.
In the final story arc Mustang and crew also subscribe to this policy, at least in regard to human enemies. However, their allies, the Briggs soldiers, do not.
In Saiyuki Gaiden the heavens have to abide by this rule and do so to varying degrees. Konzen is vegetarian, however Kenren thinks it's fine to eat fish but has to fight in the army with a stun gun...yet Tenpou has a katana but presumably still obeys the rule. Of course all of that goes out the window in the end. Averted entirely in the main series; the main characters kill many, many, many youkai on their divinely-ordered trip to India.
Roger Smith in The Big O lives by a self-imposed code of ethics. It appears to trend towards not killing people as he is very reluctant to use any form of firearm, and when pushed to use one will shoot at objects rather than people.
In Tiger & Bunny the Heroes employed by Hero TV refrain from killing criminals. However, the public seems to have gotten tired of this "soft" method of dealing with criminals. When a Vigilante Man NEXT calling himself Lunatic starts killing criminals, the public loves him.
Tate: I will not kill, nor will I assist in anything leading to killing.
Juvia of Fairy Tail admits to live by this philosophy when she fights Meredy, and she states that the other Fairy Tail members do the same. Ironically, Juvia was a former member of the rival guild Phantom Lord which members (at least most of them) didn't seem to hesitate killing their opponents. It's unknown if Juvia also had her no-killing moral code when she was a Phantom Lord member, of if she undertook it after her Heel Face Turn. Interestingly, another Fairy Tail member, Gajeel, also pulled a Heel Face Turn from being a Phantom Lord member, and when he was a villain he didn't shy away from trying to kill the heroine Lucy. He has still shown to be quite Ax Crazy even after turning good, so it's definitely not sure that he refuses to kill his opponents even though he might not do it in cold blood.
In Ginga Densetsu Weed, Weed (the Pup Hero) strongly believes in releasing enemies after they had enough. Doesn't stop him from killing a wild boar that threatens his pack, though.
Despite him beating his enemies to a pulp, Ise the protagonist of High School D×D hasn't killed anybody at all. Not even Mooks!
Code Breaker has the main character Sakura who goes to great lengths to prevent Rei and others from killing, as she believes all life is precious. However after the Re:Code arc, expect her personality to be centered around this.
This is the key point of contention between Ryozanpaku and Yami in Kenichi: The Mightiest Disciple. Yami's followers believe in upholding the "Satsu-jinken" (killing fist) philosophy, which they feel is the most original (and therefore most authentic) interpretation of all martial arts. On the other hand, Ryozanpaku's martial artists uphold the "Katsu-jinken" (merciful fist) philosophy, as they believe there is no longer any need for killing-based martial art techniques in the modern world.
In Eureka Seven, both Renton and Eureka have stop killing humans after Renton realizes what he has been doing to the LFO pilots, and when Eureka realizes the value of human life.
In the TV sequel Eureka Seven Ao, Eureka did not attack the Scub Coral despite her antagonising it, to honor her husband Renton's wish not to kill her own kin.
"In the case of Naruto and Dragon Ball Z, the reason you don't see any of the main characters take a life is pretty much the same reason most of the villains in Disney movies fall to their death offscreen, rather than killed triumphantly by the heroes. It's because we want our protagonists to remain pure. Even if it were justified, we'd rather not have blood on the hands of our favorite, pure-hearted characters. Given that Goku and Naruto themselves function as, essentially, the moral compass of their respective worlds, it makes narrative and thematic sense for them to avoid any sort of bloodshed, no matter how dicey things get."
In A Certain Magical Index, Touma Kamijo is noted for his ability to defeat bad guys and solve problems without killing anybody, and he'll even save his enemies' lives.
Dr. Orson from A Cruel God Reigns stresses this to Jeremy while he is counseling him before his death. He tells him that killing will only succeed in harming his own soul in the end. Subverted when Jeremy, while still considering this, commits Vehicular Sabotage.
In the Wildstorm Comics imprint, in one issue of The Authority, it was mentioned that an alternate earth was essentially destroyed when the Hero refused to kill their enemies no matter what, and the villains killed every single one of them. They attacked the Authority's Earth, and were quickly killed, much to their surprise, saying "Superheroes don't kill". Unfortunately for them, The Authority did. This universe in general, and The Authority especially, fall on the cynical side.
Superman has taken a solemn vow never to kill. It's strongly implied - and outright invoked in Whatever Happened to The Man of Tomorrow? - that he believes that he should give up being Superman if he takes one. (He cannot kill himself, since his oath applies to himself as well. This was shown in a number of Bronze Age stories, including one where he is caught in a hallucination that supposedly shows the future and realizes that since it shows him killing himself, it must be false.)
Superman: I broke my oath. I killed him. Nobody has the right to kill. Not Mxyzptlk... not you... not Superman. Especially not Superman.
A Post CrisisSuperman story had the hero face such a situation when an abusive husband, whom Supes gave a deserved thrashing, later murdered his wife. Superman later caught him secretly viewing her funeral and was sorely tempted to kill him right there and then. However, the relatives of both husband and wife began to plead for Superman to spare him and kill him respectively. Superman, holding the murderer while this argument is raging, painfully realized that he was in no position to make such monumental decisions and decided to simply hand the criminal to the police so the justice system can handle the matter.
Another Superman story, "What's So Funny 'Bout Truth, Justice, and the American Way?" famously had him forced to deal with the arrival of "The Elite" (a thinly-veiled copy of The Authority), superheroes with absolutely no qualms about killing villains. Over the course of the issue, he watched them become more and more popular, despite their excessive use of force. In the end, he challenged them to a fight - and proceeded (after giving them a Hope Spot) to subdue them more or less harmlessly. Though it sure dang LOOKED like he killed them, until he revealed that he used painful-looking non-lethal techniques. The story showed not only why does Superman not kill, but just how downright scary he would be if he did. The story eventually got an Adaptation Expansion into the made for DVD movie Superman Vs The Elite.
And another Superman story had him actually killing (well, executing) three Kryptonians on an alternate Earth who had annihilated all life on the planet. Despite the circumstances which almost anyone else would deem it both just and necessary—as they had committed the act of planetary murder, threatened to find an way to Superman's universe and do it again, and were stronger than he was—the act haunted him for years. After he did it, he even developed a split personality and then exiled himself from Earth after he got that under control.
This trope is somewhat justified in another story where Superman explains to the Ultramarines, a team of superheroes known for their use of lethal force, after the Justice League has pulled them out of a situation they were unable to handle, that their "'no-nonsense' solutions just don't hold water in a complexworld of jet-powered apes and Time Travel," as death apparently held less barriers for them, and in fact was more merciful, than some of the extreme incarceration punishments the League had to devise.
Superman has shown to be one of the most extreme examples of never killing. In one case he saved Darkseid's life (Darkseid helped him stop the threat that put him near death, granted, but come on, it's freaking Darkseid) and in another instance, he was trapped in a dimension where he was forced to go to war with demons for a thousand years, but still refused to kill them. He even initially objected to Wonder Woman killing them, but didn't have an answer when she asked him what she was supposed to do.
Though it's often overlooked, during his final fight with Doomsday at the end of "The Death of Superman" storyline, he was trying to kill him. If he hadn't, Doomsday likely would've destroyed Metropolis and everyone in it. It probably helps that Doomsday's mind was read a couple of times in the story, and was revealed to be nothing but rage and bloodlust. This was followed up in Hunter Prey, as Superman, after finding out that Doomsday was now far more powerful than himself, and constantly growing in might, he could come up with no other available options than letting Waverider exile the beast to the end of the universe, to let entropy consume it. Doomsday was later rescued by Brainiac, keeps coming back after being killed, and heals all other injuries instantly, so breaking his neck has the same effect as knocking a regular villain out, which, in combination with being more than Superman can handle upfront, is the reason why he can be the exception.
Kingdom Come revolved around the fact Superman abandoned humanity when he realized the public approved Magog's murder of The Joker.
In the pre-Crisis mini-series Phantom Zone Superman confronts General Zod, who laughs that Kal-El won't kill him. He's right. To quote: "I can't take your life, much as I'm tempted. But my code doesn't say a damn thing about not battering you to within an inch of it, murderer!" Once Zod is out cold for a long, long time, Superman, still holding him by the tunic, thinks "And there are times I've considered chucking that code entirely."
Played with in the Legion of Super-Heroes story where the Legion decided whether to expel Star Boy for killing. Superman voted to not expel him. The reason? Because it's easy for him, Superman, to have a code against killing when there aren't a lot of things that can hurt him, but other people may have different circumstances than him and he has no right to hold them to his code.
The Post Crisis version of Wonder Woman has trained as a classical Greek warrior with a fighting practicality of that time. That means while she is willing to control herself in combat when possible, when she decides that lethal force is necessary, she will use it without any regrets as seen when she beheaded the demigod Deimos in order to help her friends in peril.
In a crossover series, Wonder Woman cold-bloodedly executed Maxwell Lord by breaking his neck. Although some other heroes have accepted the justification (Lord had telepathic control over Superman, had killed Blue Beetle, and was at the heart of a planet-wide conspiracy), she was wanted for murder by some authorities as the act was broadcast.
In the long running independent superhero comic book, Nexus, the titular superhero kills as the very reason of his career; he periodically has agonizing dreams of the crimes of murderers that will drive him insane unless he eliminates the cause by going out to kill the criminals and he has the power to get through nearly any defense to do so.
In recent years comics have tended towards a greater degree of cynicism, with the result that most heroes have ended up killing at least once. Probably the only major character who hasn't been forced to it at some point is ironically one of the most gritty heroes, Batman.
Which is peculiar when you look at Batman's first appearances back in Detective Comics where, in true pulp style, he kills the bad guys when circumstances require, though he is shown as being reluctant to do so. It's not till the boy hostage shows up as the first teen sidekick that Batman begins to soften and not until he kills some giants (with guns no less) in the first Batman comic that the decision that he stop killing (and using guns) is made. Not by the creator, you'll notice.
Final Crisis has finally shown us the only person so evil and dangerous Batman was willing to kill him: Darkseid. Which just makes the Superman example above even funnier. By the time Batman kills Darkseid the villain has taken several levels in badass, and was destroying the entire universe just by existing. Whereas Superman saved an alien warlord, but this point it's one life against all of time and space.
Batman has outright stated that he's tempted to take lives all the time, but doesn't because he knows that if he finally takes one, he's NOT going to stop there and probably never would stop.
In the Novelization of Knightfall, Bane mocks Batman as not wanting to kill despite his dark outfit. Batman snaps back, "Don't count on it."
Flashpoint Batman aka Thomas Wayne dispenses with this altogether and shows himself to be an exceptionally violent and uncompromising psychopath who has killed off a good portion of the villains in that timeline and threatens to use lethal force all the time.
The Legion Constitution was once published in the comics, in its entirety. The section in question says that "[n]o Legionnaire shall take the life of any sapient being, save as a provable only alternative to the death of the Legionnaire, or the deaths of other sapient beings." The writers usually have the Legionnaires treat the question of lethal force more strictly than their constitution actually requires.
Robot: Keeping them here would be a death sentence. Hopefully they've learned their lesson.
Invincible: Right... and I'm supposed to be the new guy.
Robot: It is not mathematically inconceivable that at some point we encounter an adversary that realizes the error of their ways and gives up their plans for revenge.
Invincible: I hope you're right.
Daredevil used to have a typical view of killing, claiming that it wasn't his place to pass judgment. During Frank Miller's run, which redefined the character, Daredevil eventually went against his principles when he tried to kill his archenemy Bullseye. He's killed people several times, and he hasn't tormented himself for issues on end because of it, perhaps the only "regular" superhero who can make this claim.
However, despite the occasional team up he is frequently at odds with The Punisher for his blatant disregard of the no-kill rule, to the point where the latter might qualify as a members of Daredevil's Rogues Gallery. Though their enmity has softened somewhat ever since Punisher rescued Matt from prison and helped him keep his secret identity, Daredevil was probably the hero most devoted to locking Frank Castle up, even more than Spider-Man who hardly ever tried to capture him. Murdock was known to organize hero teams for the sole purpose of hunting The Punisher down.
The key word here being "blatant." Daredevil has killed when the situation called for it. And when the situation has called for it, he has hated but not regretted doing it. That said, he does not endorse wholesale murder as the answer to his, or anyone else's, problems.
Well and truly averted after he finally killed Bullseye.
Black Panther is not adverse to killing, though he usually tries to use non-lethal means if at all possible. During his run as the protector of Hell's Kitchen, he notably told a thug that as a warrior first and foremost, he did not share Daredevil's no-kill rule.
In The Avengers, there is (was?) a very strong policy against killing, to the point that one of their mottoes was "Avengers don't kill." This has been brought to attention several times, with Hawkeye almost getting separated from his wife because he heard that she allowed her rapist to fall to his death.
Their later divorce was specifically built on the tension caused by this incident. Notably, however, all the Avengers who heard Mockingbird's side of the story (Hawkeye heard about it from the ghost of the dead man, who significantly downplayed his actions) sympathized with her, because her circumstances were considerably different from that of their usual fights.
These days they are a little more flexible about this rule.
This was continued in Secret Avengers. In one instance, Beast was forced to kill a group of terrorists in order to save the populations of two large cities. He was understandably upset by this, and Captain America comforted him by telling him not to dwell on the few lives he'd taken, but the millions he'd saved.
Spider-Man is also strongly against killing anyone.
In Spider-Man Noir, Peter carries a revolver in his masked identity, and uses it to kill the Vulture when he threatens Aunt May. The fact May is as horrified by him as the Vulture is what convinces him to adopt his mainstream counterpart's morals.
In Superior Spider-Man, Massacre is executed by the new Spider-Man (Otto Octavius) after going on a bloody killing spree. Wolverine defends Spider-Man's actions by noting that Massacre was a particularly vile and depraved villain, and that most of the Avengers have used deadly force at one point or another.
In X-Men, the rule against killing is partially due to the usual reasons, and partially due to human/mutant relations. Mutants have a hard enough time withoutWolverine carving people up on the six o'clock news, so you'd better stifle any Darker and Edgier tendencies, especially while wearing an X symbol. However, it's not as absolute as it is with Batman or Superman, as individual members can fall anywhere from The Cape to Nineties Anti-Hero, and most X-teams will defend themselves or others lethally if it's the only way. A few of their main villains also have Joker Immunity.
In one issue, Cyclops explicitly refutes this trope with regard to villains over Storm's objections when they're looking for a villain who may have perished in a fight with the team; he states that he doesn't take killing lightly, but at the same time isn't going to waste any tears over someone who poses a clear risk to his team and students and has no compunction about attacking and killing them.
Nineties anti-hero Cable (an amazingly powerful telekinetic infected by a nano-technological virus who used huge guns ... no, really) had no qualms about killing and invariably racked up a huge body county every issue. Always without any ramifications. And in his most recent shared series, he came across as the good partner. The other guy was Deadpool.
Batman is the poster boy for this trope. In fact, it's been heavily implied that his almost psychotic compulsion to never kill is the only thing keeping him from being one of the psychopaths he regularly fights.
While he won't actually kill, Batman is generally more than happy to go as close as possible to the edge, without actually crossing the line. This is demonstrated at least once during The Dark Knight.
Salvatore Morone: "From this height, the fall wouldn't kill me."
Batman: "I'm counting on it."
Cassandra Cain/Batgirl II had an even deeper aversion to killing as she could read human body language perfectly. After seeing death once she vowed to never see it again and tried to save a death row inmate to uphold that oath.
Invoked to an almost headache-inducing degree in the early 2000s run of Justice Society of America. Black Adam, having gotten utterly fed up with villains who don't give a damn about the lives of people being allowed to go free again and again, gathers up a small crew of like-minded people and goes off to smash the brutally dictatorial regime that's set itself up in his home country. Even though one (one) JSA member acknowledges that they and the US government had turned a blind eye to the fact that these people had been conducting murder sprees and enslaving children, the entire team nonetheless goes after Adam's crew for taking them out. And then when Hawkman's methods for dealing with Black Adam's allies proves too brutal for their taste, they turn on him. All in about five issues.
This is actually one of the few times both sides are given fair consideration. Some Green Lanterns are against it, some are all for it, but neither side is presented as wrong and the ones against killing can't deny that being able to kill was the main reason they won the war. (Though needless murder is right out.)
The Sonic the Hedgehog comic has something like this: in one issue Dr. Robotnik, still insane after the events of issue 200, is locked up in New Mobotropolis. A character asks Sonic why he's showing mercy to Robotnik. Sonic admits he doesn't know for sure, and guesses he moves too fast to get hung up on revenge. The character isn't sure if Sonic has a Zen state of mind or is foolish, but he's impressed either way.
In issue 225 Robotnik mocks Sally for showing mercy on all the times she could've finished him, as doing so allowed him to stay a threat. A few pages later, he seemingly killed her, then reset the Universe.
During Dark Reign the Thunderbolts team observed that, despite being Osborn's hit squad, they almost never killed anybody. In fact they completed one assassination without taking any lives.
Used for great dramatic effect in ElfQuest. The main tribe of the story, the Wolfriders, have one simple rule: elves don't kill elves. It's a concept so ingrained in their culture, killing others of their kind would not even occur to them. Until one elf from a different tribe, Kureel from the Gliders, ends up kidnapping a young Wolfrider and threatens to kill him. The boy's father (the tribe's archer) shoots and kills Kureel. He goes into a complete Heroic BSOD until he's finally able to ask Kureel's spirit for forgiveness many months later.
Interestingly, at the very start of the series, the Wolfriders seem to treat death much more casually, briefly considering killing Rayek because he looks at them funny. This is, however, shortly after the humans burnt down their home and the trolls betrayed them and left them to die in the desert, and while they were still figuring out what to make of these strange new elves who walked around in broad daylight, had huts, and actually cooked their meat just like their old human enemies did. (It's telling that the Wolfriders decided to 'introduce' themselves to the Sun Village by raiding it for food rather than just walking up and saying hello. Thankfully for both sides, that didn't last long.)
Also interestingly, in the very early days of the Wolfrider tribe, there were many elves born with wolf-blood and just as many wolves born with elf-blood (it's not as icky as it sounds - the first elves were shapeshifters lost on a low-magic planet. Mating with the local fauna was their way of bonding with the new land). Timmorn, the first Chief and first mix between elf and wolf, took on the task of deciding what was elf, what was wolf, and what should just be killed instantly.
Two-Spear didn't have too many qualms either about killing his own daughter. But (a) Two-Spear tried to be more wolf than elf, using the pack's way of life as an excuse to act violently insane, and (b) the story in which he thinks he's killed his daughter was a case of Running the Asylum anyway.
Obviously, The Punisher has no business with the standard version of this. However, he will absolutely under no circumstances ever kill someone who isn't a criminal or otherwise corrupt. He'll go out of his way to prevent bystander casualties and will even let a bad guy slip if he has to. (Depending on the Writer. At least one issue has him willingly allow a woman to be killed in order to stop a criminal who is banking on his "Doesn't allow innocents to be harmed" schtick.)
In Young Avengers, Hawkeye (Kate Bishop) notes that the Punisher is considered a Serial Killer by almost all of the superhero community by now, and most would bring him down if they got the chance.
If recalling correctly, the woman the criminal used in the above-mentioned scene was a willing Judas-Fish who did whatever the criminal said and gleefully helped to put Castle in the trap of the main baddie. Had she been a true innocent, Frank might well have moved to help her. As it stood, she died while painfully realizing just how nuts her boss was. In fact, the scene (in some kind of Summer Special Issue, I think) is reminiscent of The Incredibles, where it went just a tad better for the henchwoman, Mirage. This special came out ten or more years before that, though.
With the exceptions of truly mindless incarnations of the character, the Incredible Hulk rarely kills anyone intentionally. Most deaths caused by his rampages are accidental and the result of property damage, that—to be perfectly fair— could result from most superhero battles (admittedly, the Hulk tends to cause more damage than most superheroes). Even then, deaths are fairly rare. In one issue, where Bruce Banner admits to murdering his abusive father and making it look like an accident while defending himself, he stated that as the Hulk, he had leveled entire cities without killing a single person. All of this being said, it isn't clear just how much of this is intentional and how much is coincidental; in some cases the Hulk clearly intends to kill an enemy, with them happening to meet a Karmic Death during the course of the battle.
In Empowered, this is played utterly straight with the title character; even her most powerful energy blasts have never been seen to do worse than leave someone out cold. The rest of the cast (including, from the look of things, her costume) averts it hard, especially Thugboy. In volume 6, she does leave Deathmonger to be disintegrated by a nuclear blast... but he's not only an enslaver of the walking dead, but a walking dead man himself.
Birds Of Prey member Huntress had no time for this early in her vigilante career. She's getting better, but she still doesn't seem to have too much of a problem with killing criminals. It's the main reason Batman doesn't trust her. Oracle, being more forgiving and willing to offer second chances, does trust her. Oracle does, however, use this excuse to treat her like crap.
Green Arrow is a big believer in this. It's why he uses so many trick arrows, like the infamous boxing glove arrow, instead of actual arrows. The downward spiral that culminated in his first death started the night he actually killed someone. He made an exception for Prometheus after the latter attacked Star City with a Kill Sat and killed thousands, including his granddaughter Lian Harper.
Seriously averted during Mike Grell's run, where Green Arrow began using lethal force regularly after killing a man who was torturing Black Canary. The series flip flopped on how he felt about killing, sometimes doing it casually and other times feeling remorseful about it. Once his series ended, the events and characterization have been ignored.
Enforced in Quantum And Woody by Quantum's heroic idealism. This proves problematic when the duo attacks Terrence Magnum's private mercenary army and Quantum has loaded Woody's rifle with rubber bullets.
In Antarctic Press' Gold Digger, the giant superheroine Crush is adamant about this - mainly because, during a brief period during which she was being blackmailed by a supervillain, she killed a bunch of gang members... and, coincidentally, an undercover cop.
Moon Knight is a strange case. Being Batman wearing white and an obsession with Egyptian moon gods of vengance, he has a disdain towards killing. However, it's not so much he doesn't want to be like the people he fights, it's that he is extremely tempted, to the point of addiction, to killing, and wants to fight it. Doesn't stop him from torture, maiming, and cutting off a guy's face and wearing it for the sake of the moment.
The Teenage Mutant Ninja Turtles, being ninja, were pretty much trained to kill from their first appearance in order to avenge Splinter's owner via his death from The Shredder. It varies from incarnation to incarnation with some versions vowing never to kill, but for the ones that do it's made apparent that none of them really relish having to kill anyone period, but if it comes down to survival or saving someone they will take that step.
Discussed in Issue 9 of The Shade (2011). The Shade is about to kill a villain before he brings up this trope. The Shade outright denies being something so "average". The villain then talks to Silverfin, a friend of The Shade's and a true hero. Silverfin then responds that, as a hero, he only fights for what he perceives as good, citing no superhero rulebook. And if letting this villain die is a good thing, then he'll let it happen.
Subverted in Prince of Persia: The Graphic Novel. Guiv subdues a lion which attacks him in the mountains. Kneeling over the lion, he lifts his sword, then has a thought and says, "No one gave me the power to take life." As he walks away, the lion gets up and charges at him, and he quickly turns and slashes it to death.
Turul: No one gave you the power to spare life, yaaahr.
In the Justice League/Naruto crossover "Connecting the Dots", the superheroes have a very hard time getting the ninjas (who are essentially underage soldiers) to understand this concept.
One scene that stands out is where Robin and Cyborg are for this trope, Neji and Sakura are against and Naruto as devil's advocate after Neji killed Dr.Light. Just a taste of that talk:
Robin: "You killed him."
Robin: "We don't...you weren't supposed to do that?"
Neji: "He was required for interrogation then? My apologies. I assumed termination was the objective"
Robin: "Termination is NEVER the objective! Not with us! We don't kill!"
Neji: (puzzled)"But the fight necessitated the need for lethal action. Cyborg (*points at him*) knocked him off a 10 story building with intent to kill."
Cyborg: "Well yeah, but… Dude, we knock people off buildings all the time! They just never actually fall, they always catch themselves!"
Neji: "So it was a distractionary measure? But I thought you didn't know he had that jetpack. And Beast Boy destroyed it shortly after, he could have died from either of those falls."
Cyborg: "Dude, the point is that a fall NEVER actually kills anyone."
Neji: "Yes they do. I've killed several people that way."
Robin: "The point is that Beast Boy and Cyborg didn't aim to kill him. You did."
This is normally a point used in Harry Potter fanfiction when mocking a bashed Dumbledore, who he and his followers tend to view any sort of death as immoral and dark. For instance, in the story Harry Potter and the Curse's Cure, he is horrified by Hermione's fanfiction made uncle, Dan Granger's, killing of several death eaters with a gun (Despite the fact that they were there to kill and rape Hermione and her family). In the story The Harem War, he refers to Harry as dark when he kills Dolores Umbridge and two death eaters with a gun, despite the fact that the universe that Harem War exists in explains that Death Eaters have to kill in cold blood and rape to join, and the fact Dolores sent a squad of them into the innocent country that Harry ruled (of course, this Dumbledore is evil however).
The four in With Strings Attached, being Actual Pacifists, are very much committed to this stance, sometimes to the point of having to get really creative to solve a problem because the opponent's death, or even the opponent's ass-whooping, is not an option. The irony is that collectively they have been gifted with enough power to wipe out a city before breakfast.
Though at least two of them were not averse to handing out a good nonlethal ass-kicking in revenge for heavy abuse (of themselves or their True Companions) at the hands of some baddies.
Plasmas Folly's interpretation of N fiercely opposes killing after having to witness the deaths of many of the injured Pokémon Ghetsis gave him to play with as a child. This becomes a bit of an issue when he realizes that putting the suffering Genesect to death may be the best option.
The Tim Burton/Joel SchumacherBatman movies have been a bit more flexible with this trope than the comic book version, with Batman demonstrating that he's not especially concerned if his enemies end up dead on numerous occasions. The Christopher Nolanmovies, however, have been a bit closer to this trope, with Bruce Wayne's refusal to kill a key element of his motivation ("That's why it's so important. It separates us from them."). However, in Batman Begins, he informs Ra's Al Ghul that "I won't kill you... but I don't have to save you.", before flying off, leaving Ra's in a train car that soon after crashes and explodes, presumably killing him. Anyone who knows Ra's from the comics knows it's a case of Immortal Life Is Cheap, even if Batman doesn't.
In Batman Returns, he gives a clown a bomb, then smiles sadistically as the guy is blown to pieces. He enjoys killing in Burton's films.
By The Dark Knight his moral philosophy appears to have evolved somewhat, as towards the end he goes out of his way to save The Joker's life. On the other hand, the Joker was trying to drive Batman to murder, so this looked like the only way to beat him.
He also has another justification besides personal philosophy: he's a Hero with Bad Publicity in the Nolan films, so he knows acting as judge, jury, and executioner isn't going to help his reputation.
Another fact to consider is that Batman personally threw the Joker off the building. If he didn't catch the Joker, then he explicitly killed him. But with Ra's, Ra's willingly put himself on the train with the knowledge that Batman would try his absolute hardest to stop him. Ra's taught Batman everything he knows and remembers that one time that Bruce unintentionally burnt down an entire fortress to avoid killing. Ra's obviously understood the potential risk of going against Batman, and one could reasonably assume that Ra's would have some sort of way to escape. Nolanverse's Batman follows the could that he will never intentionally kill a person, but if the badguy puts himself into a position where s/he will be killed by collateral damage in the act of Batman saving Gotham / the innocent, and there is no way to save them, then there is nothing that can be done. Ra's had no way of saving himself on the mountain; but, Bruce could save him, and so he did. On the train, Batman had reason to believe that Ra's could save himself, and the only choices were Btaman and Gordon destroy the train, or every living thing in Gotham dies. The same exact problem comes up in The Dark Knight Rises, when the nuke will go off in less than ten minutes, the tanks are actively trying to kill Batman and Catwoman, they cant force the truck to go back to the generator, and all warning shots have failed to get the truck to stop. Either the truck and tanks are stopped with force, or literally everything in Gotham is wiped off the face of the earth and the rest of the US gets hit by the fallout.
In The Dark Knight Rises Batman explicitly tells Selina Kyle "No guns, no killing.". She is less than enamored with the idea, responding, "Where's the fun in that?!" Selina later saves Bruce's life by shooting Bane right as he is about to kill the hero, and jokingly states that she doesn't feel too strongly about the whole no-kill thing.
This is in comparison to Batman: The Movie when he was trying to find a safe place to dispose of a bomb he refused to throw it where anybody could get hurt. Including at ducks. Later in the movie when he and Robin accidently kill some mooks they do mourn for them as they weren't expecting them to combust.
One of the three "Genie Rules" stated by the Genie from Aladdin prohibits him from killing anyone. Kinda sucks for Jafar... On the other hand... Jafar doesn't have any real problem with the rule, since he can still invoke a fate worse than death in either case or cause circumstances that result in a person's death as long as he isn't the one directly pulling it off. A Running Gag in the movie is, after mentioning that Genies can't kill, someone says "You'd be surprised at what you can live through!"
Butch Cassidy, the tough, notorious, wildly-successful, train-looting, bank-robbing, gun-waving, badass outlaw, when faced with the prospect of a shootout, lamely admits to the Sundance Kid that he had never killed a man in his life. It's almost painful to hear Butch pleading with the bandits to go away so that he won't have to defend himself.
Ironically, this is just after the two of them have quit their criminal lives for a legit job.
In Warriors of Virtue, the Warriors cannot kill. In fact, when Ryan arrives, he learns that their leader, Yun, is in the middle of a Heroic BSOD because he accidentally broke that code in the heat of battle. The fact that the soldier he killed was actually Elysia's brother probably didn't help his mindset much.
Subverted in Mystery Men. The Bowler, a woman whose bowling ball has her father's spirit within, confronts her father's killer. He taunts her, saying that she doesn't have the nerve to take her revenge. He is right; she's a hero and as such above that. Her father, however, is dead and pretty pissed about it and is something of a prick, so he really has no problem killing the guy.
But played perfectly straight with Dr. Heller—which makes the heroes reject his help, until he shows them how effective his inventions can be.
...where both John and Sarah try to live by this, but over the course of the series are forced into taking a life each (see below).
In Blue Thunder, protagonist Frank Murphy is a police helicopter pilot, and he naturally goes to some lengths not to kill anyone (except the Big Bad) even while they're trying to shoot him out of the sky. This despite being in command of a heavily armored Black Helicopter armed with a 20-mmrotarycannon, which has an uncanny ability to blow away cars, choppers, and aircraft without harming the people inside.
In Hot Fuzz, Nicholas Angel aims for incapacitating shots in the final shootout. Despite the several gory murders before, the final shootout sees no deaths.
Angel's shooting skills were purposely laid out as a Chekhov's Skill early in the film, so it's justified. Danny, on the other hand...
Recited verbatim by Brother Gilbert in Dragonheart before deciding to kill the evil king Einon.
Surprisingly averted in The Adventures Of Captain Marvel, where the titular hero kills no less than 3 people over the course of the 12 chapter film serial. Given this take on Cap was more of two fisted pulp adventurer than a traditional superhero it makes sense, and he does spare the lives of most of the villains he faces.
Megamind knew that Metro Man went by this rule, always sending him to jail. It was one of things that made their battles seem like a game to him. Unfortunately, when he created Titan to take Metro Man's place, he didn't anticipate Titan making his own rules.
Subverted, in a rather surprisingly brutal way in the movie Darkman. The protaganist has caught the bad guy from falling to his doom by the pantleg. The bad guys starts into a typical "You can't kill me, you're the good guy..." speech, and unwisely ends it with the line "you couldn't live with yourself." The protagonist, who by this point has been burned beyond recognition, left for dead, and surgically altered, promptly lets go of the bad guy, letting him fall to his death, replying "I've had to learn to live with a lot of things." Given that the protaganist was played by Liam "I will find you and I will kill you" Neeson, he really should have known better.
Technically, The Bible doesn't say "Thou shalt not kill", it says "Thou shalt not murder/shed innocent blood". In other words, don't kill someone without a very good reason. Warfare and capital punishment were accepted practice in ancient Israel and in many cases sanctioned by God.
Killing in self-defense of your own life, or killing someone who is currently attempting to murder someone else, is likewise permissible and obligatory under Biblical law, as elaborated in the Talmud; a would-be murderer is considered legally "dead" for the period of time in which he is attempting to commit murder, and killing them during that period is therefore not considered murder at all. (It is still preferable to stop them in another way if possible.)
In the New Testament, Paul quotes that commandment as "Thou shalt not murder". This disagreement might have something to do with the fact that not only did the translators write in an outdated style (for the day, that is), not all of them spoke English on a regular basis. It easily meshes with Christ's teachings as well when you think about it: He said to turn the other cheek when struck, not to offer up the other kidney when stabbed. Sufficient to say, like many parts of the Bible, there are other interpretationsto what this actually means.
The greatest warrior is he who does not need to kill
In J. R. R. Tolkien's The Lord of the Rings, when Gandalf tells how Mordor has learned from Gollum that the One Ring is now in the possession of hobbits, Frodo exclaims, "What a pity that Bilbo did not stab that vile creature, when he had a chance!" Gandalf admonishes him:
Gandalf: "Pity? It was Pity that stayed his hand. Pity, and Mercy: not to strike without need... Deserves death! I daresay he does. Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgment. For even the very wise cannot see all ends."
This really comes down to the difference between killing an enemy in battle (which neither Frodo nor Gandalf shows any aversion to) and executinga defeated foe. And as it turns out, Frodo fails in his quest to destroy the One Ring, with Gollum completing it for him... by accident. Which could tie into some translations of the trope-naming commandment using "murder" instead of "kill".
The same mercy is shown to Gríma Wormtongue (twice!) and Saruman as well. Neither can comprehend mercy, thinking it's a trick. As a contrast, Gríma kills Saruman at the end.
This is spoofed in the prologue to Bored of the Rings, where, after the answer to "What have I got in my pocket?" is demonstrated to be a .38 pistol, the thought behind "pity stayed his hand" is explained as "It's a pity I've run out of bullets."
The Seekers Of Truth use this, as they work with the law enforcement and justice system. A couple of them violate this rule once, which as it turns out is one time too many.
Interesting subversion in Warrior Cats, where the warrior code says: "An honorable warrior does not need to kill other cats to win his battles, unless they are outside the warrior code or it is necessary for self-defense.", so Thou Shalt Not Kill... unless it's in self-defense... or the person you're killing really deserves it. But you are still just considered "dishonorable" (although, being Proud Warrior Race Guys, this is A Fate Worse Than Death for some). The rule is still important, though, and main characters have so far only killed Big Bads, and at times have had to be restrained from killing others.
Pulp hero Doc Savage started out killing bad guys left and right, but evolved a pragmatic "don't kill unless there's no other way" policy after the first few stories. Many a villain ended up fatally Hoist by His Own Petard. More of than not, Doc knows this is going to happen (since he's sabotaged the weapon) and tries to warn the Big Bad, who just laughs and pushes what has now become the Big Red Button.
An alien race in Tom Holt's Falling Sideways had this as a rule. They also had a very high level of technology and the collective mindset of a Rules Lawyer. As in, it's OK to make people believe themselves to be frogs and eat nothing but flies, because they have a rule saying "Thou Shalt Not Kill" but not "Thou Shalt Not Make People Feed Themselves Horribly Inadequate Diets".
In The Mysterious Benedict Society books, Kate's father, Milligan, always works to find solutions that would avoid killing his opponents (generally the vicious Ten Men) no matter how savagely they try to kill him or others. When asked about this by his daughter, Kate, he tells her simply "We're not like them." Indeed, when Kate later has the opportunity to toss a bomb at them and their leader, Mr. Curtain, she instead tosses it away into the water where it can do no harm.
In the Harry Potter series, the rules here are… tricky. Wands are often wielded threateningly like guns, yet the actual Killing Curse, Avada Kedavra, is extremely illegal, and using it possibly requires some degree of malice. (Perhaps not in the case of Mrs. Weasley, although Bellatrix was threatening to kill her daughter, and almost certainly not for Snape's mercy-killing Dumbledore). Nonetheless, there are numerous other spells (like Sectrumsempra) which would presumably also cause death under the right circumstances. In Book 7, the disarming spell, Expelliarmus, becomes Harry's pacifistic trademark, and the following conversation occurs:
Lupin: "Harry, the time for Disarming is past! These people are trying to capture and kill you! At least Stun if you aren't prepared to kill!"
Harry: "We were hundreds of feet up! If I Stunned him and he'd fallen, he'd have died the same as if I'd used Avada Kedavra!"
The magical world apparently has extremely dim views about killing, since murder can literally rip your soul in half.
Artemis Fowl tends to avoid any killing by the good guys, regardless of possible need or justification. No character at all died in the first book, and the only deaths in the second were three goblin assassins, one by Karmic Death in an avalanche and the other two shot in the back by their accomplice, as well as the Big Bad's second-in-command also by accidental Karmic Death.
In The Dresden Files, the First law of Magic specifies that Thou Shalt Not Kill With Magic. Violating this law generally leads to execution by the White Council, except in rare cases where the wizard responsible was judged to be acting in self-defense and another wizard is willing to mentor the killer. Killing people without magic is allowed if circumstances dictate though. The sole exception to this is the Blackstaff, who is allowed to kill if it is deemed absolutely necessary. Also, the Law specifically states that it is illegal to kill humans. Killing supernatural creatures such as faeries and vampires with magic is allowed, as the Laws only exist to protect humans.
Technically, the law is to not use black magic, which includes mind control and the like ( Hence Blackstaff). It gets into a grey area with regard to collateral damage from magic and largely involves intent as being a key factor, thus making it closer to Thou Shalt Not Murder. Because of the nature of magic in the Dresdenverse, a person must believe in what they are doing. So using black magic and murder actually taints the soul and is very addictive. For what it is worth The Blackstaff is one of the kindest people in the series, and when he starts unleashing magic to kill it is a horrifying scene.
In the Iron Druid Chronicles Druid magic cannot be used to harm a living being in any way. If you do, the magic will kill you on the spot. However, Druids can kill people in any number of mundane ways like cutting their heads off. The prohibition also only applies to direct magic use. Druid magic can be used to indirectly hurt someone (eg summoning elementals to do the fighting or simply having a hole appear in the earth so the opponent breaks a leg). It also does not apply to supernatural beings with no connection to the earth (eg demons).
In Wearing The Cape Hope's expectation is that superheroes follow the Golden Age superhero code, and this is strengthened by Ajax' statement that "heroes don't use guns." But in her first fight she discovers that Atlas is perfectly willing to let the bad guys kill each other, and in the surprise-attack on Whittier Base half the team breaks out automatic pistols, the better to cap their attackers. Hope herself kills an unspecified number of terrorists in the heat of combat, then kills two heroes in the Dark Anarchist's secret base.
In the Prydain Chronicles, Lord Pryderi taunts the enchanter Dallben, believing that he "secret to his power" is that Dallben cannot kill. Dallben says he has never killed anyone, but that doesn't mean he can't.The issue is never settled, since Pryderi dies shortly thereafter without Dallben's intervention.
In Septimus Heap, Aunt Zelda has to remind Nicko of this when he suggests to make the Hunter remember he's not a lion tamer while he has his head in a lion's mouth.
In The Quantum Thief, the Sobornost collective hold a rare, villainous principle in this matter. They could wipe out all their enemies from the Solar System with Strangelet bombs in a matter of hours, but in their ideology, every mind has its place and every memory is worth preserving. It's just happens to be that they decide what place each mind should have, and they'll cheerfully Mind Rape the uploaded personalities into any function they find the most suitable, be it an infiltrator or a missile guidance system, and then copy them as needed. In their minds it's only a murder if all the copies of an uploaded individual are destroyed.
Trapped on Draconica: No matter who it is Daniar will not kill them, though some people really ask for it. She just about killed Zarracka after the third time she was spared. Interestingly, Rana doesn't persuade Daniar out of killing Zarracka out of concern for the villain's well being but to prevent Daniar from breaking her code.
During Galaxy of Fear, the protagonists will destroy basic speechless droids and squish beetles forming swarms, and Zak once accidentally kills a birdlike animal and feels remorse, but anything smarter, if it dies, dies some other way. In Spore, Tash flees Spore, which is controlling the crew of a Star Destroyer and taking it after her into an Asteroid Thicket, where it's attacked and destroyed by space slugs. Her Actual Pacifist friend, who flew with her, is horrified and feels like she's killed the crew. Tash says it's not the friend's fault, she was following Tash, but also puts the blame on Spore.
Similar to the Warrior Cats, Percy Jackson and the Olympians mildly subverts the trope. While the demigod heroes do kill monsters, Death Is Cheap with them. They recover in any time between weeks and centuries. On the other hand, Percy deliberately avoids killing other demigods, though it's a bit like Harry Potter's example in that many of them are duped. The views of other demigods aren't really as known but probably were not as merciful.
Live Action TV
Buffy the Vampire Slayer: Slayers are definitely not supposed to kill humans (even villainous humans). Vampires and other assorted demons are fair game. Although there were a number of Karmic Deaths for the human enemies. There was also that time she had to kill about ten of the Knights of Byzantium to defend herself and her sister, one by throwing an axe into his chest at pointblank range.
After Faith accidentally kills a human with a wooden stake while still on a massive adrenaline rush after a fight, seconds after Buffy tries to warn her Giles actually tells Buffy that due to the high-stress nature of the Slayer's job, the Watcher's Council expects one or two accidents and has ways of dealing with them. This doesn't stop the angst on the part of the accidental murderer, though. An episode soon after shows Buffy stopping just short of the killing blow to a vampire after Willow cries out, in exact parallel to the situation with Faith, showing that Faith could have avoided killing the deputy mayor if she had a clearer head. Shortly afterwards, Faith's poisoning of Angel drove Buffy to nearly kill her so that her blood could be used as an antidote. Thankfully, Faith was only put in a coma and Buffy snapped out of killer mode, though actually entering it in the first place haunts her for a good long while.
The same rule also generally applied to Angel, though he had quite a few "exceptions" to it throughout the series' course. Humans were excluded if they had supernatural powers. Even then there was the episode "Conviction" where Angel killed a special ops soldier who was technically his employee by kicking him causing him to shoot himself in the head, just to make a point that the soldier's way of doing things wasn't going to be tolerated anymore under Angel's management.
Heroes: Matt Parkman had ample justification to kill Emil Danko, who is heading a program started by Nathan Petrelli to round up persons with special abilities. First, Danko's operatives shoot Matt's girlfriend Daphne. Then when Danko takes control of the operation he removes the still-living Daphne from the medical facility. Consequently, Daphne develops sepsis, leading to her death. Parkman seeks to get even by taking away the most important person in Danko's life, a call girl named Elena who knows Danko as "Jakob Pradasa". He telepathically forces Danko to divulge his true identity, to admit what he does (hunting and abducting people), and to confess that he let Daphne die. Parkman then points his gun at Elena, but cannot bring himself to shoot her.
Supernatural: Hunters are supposed to kill evil monsters and protect humans, but just like Buffy example above, killing humans is not acceptable. The one time the Winchesters considered killing a human, Well-Intentioned Extremist Gordon Walker (who thinks Sam is purposely evil), he gets turned into a vampire (his specialty) so that Sam can kill him without feeling really guilty about it.
However, when up against truly monstrous humans (like the family who hunts other humans for sport or the other one with feral children who were bloodthirsty Tyke Bombs), Sam and Dean have had to kill. As Dean said "Demons I get, humans I don't."
The Sarah Connor Chronicles starts off with John and Sarah like this. Derek and Cameron, not so much. Sarah frequently orders either or both of them not to kill (they tend to take it under advisement). The first season features Sarah's reluctance to kill a man she believes will one day create Skynet, and is shown dwelling on it. A common theme throughout the series is the importance of human life. However, Sarah ends up being forced to kill a man midway through season two and John even earlier.
This is the Doctor'sapparent modus operandi. Give him points for effort, but it doesn't usually work.
Heavily subverted in the seventh season episode "Dinosaurs on a Spaceship". Sure, that guy had committed genocide on an entire shipload of refugees and tried to enslave queen Nefertiti of Egypt. Being used as a missile decoy was the least he deserved, but coming from the Doctor that was cold.
In Smallville, Clark Kent refuses to kill enemies, but he does not consider Karmic Death or accidental death to be murder. The one time he attacked an opponent (Titan) with the intent to kill, he was haunted after he did the deed. Chloe also stresses this often, sometimes to meteor freaks who aren't bad at heart. Oliveron the other hand... It leads to clashes between him and both Clark and Chloe. He often tries to get them to do what he does.
The Leviathans of Dark Shadows have this as a rule. Not due to any sort of morality, but rather because anyone they kill will become a Badass Abnormal ghost, capable of hindering their plans even further. At least that's how it's supposed to work, but due to Real Life Writes the Plot issues the matter was rather derailed.
In Have Gun — Will Travel, Paladin will avoid killing if possible, and more than one episode ends without anyone dying. When it becomes necessary, however, he won't hesitate.
Tracker Cole does not kill; he just incapicates the human long enough to withdraw the life force from the body. Justified, because Cirronians are a peaceful species by nature, and abhor violence (yes, even the criminals-most are in prison for nonviolent crimes).
The Lone Ranger, in some ways a precursor to Vash, used silver bullets as a symbol of his pledge never to take human life.
GURPS has the Pacifist disadvantage, which comes in several flavors, one of which is Cannot Kill. Characters with the "Cannot Kill" disadvantage can start fights and use any tactics they like, but they cannot kill, or be responsible for a death, or leave a wounded enemy to die. They also cannot stand by while their teammates administer the Coup de Grâce. If they do, they angst about it for days and are effectively rendered useless to the team.
Some Superhero RPGs would invoke rules against killing. Two notable examples were Marvel Super Heroes and DC Heroes, which would eliminate all Karma/Hero Points (a combination of experience points, and self-boosting reserves for various tasks) and keep you from accumulating more for the rest of the adventure (usually one night of gaming). In DC heroes, this punishment came from using lethal force at all.
Dungeons & Dragons has the Book of Exalted Deeds, which contains the feat "Vow of Peace". It grants benefits as long as you don't inflict lethal damage, allow an ally to finish off a defeated opponent, or cause similar harm to a creature. It takes this trope to extremes; accidentally swallowing a gnat in your drinking water will cause you to lose the benefits of the feat, in fact the feat specifically references paladins drinking their water through a strainer. It doesn't really make you a pacifist, technical or actual, though; you can still fight all you want, as long as you never inflict lethal damage.
Anyone with Compassion 5 in Exalted. If you want to finish a defeated opponent, let someone die, or let someone else do the above, you have to test compassion and fail the test. Oh, and if you succeed on the test, you have to choose between spending willpower and gaining limit to ignore it, or saving the person. Even if they were trying to kill you, or end the world. Even if you have to fight an ally. The only possible exception is killing the Neverborn, considering their suffering is bad enough to break reality. On the upside, as long as what you're trying to do will save people, compassion is by far the most flexible virtue.
"Code vs. Killing" is one of the most commonly seen Psychological Limitations in Champions, usually bought as "total commitment" (i.e. the character can't bring him- or herself to kill at all and won't stand idly by while others do it either). Normal people are already assumed to be "reluctant to kill" by default (being Ax Crazy would be its own different Limitation); the code, if taken, is intended to go beyond well beyond that to proper comic book levels. Of course, being a Limitation that you get points for, it's also supposed to cause your character trouble from time to time.
The MMORPG City of Heroes takes this to its natural extreme: the player-characters are always sent to "arrest", "defeat", or just plain "stop" the villainous NPCs, and even if the enemies are "arrested" with a high-powered assault rifle, a broadsword, or repeated fireballs, nobody ever dies. Instead, they're sent to the local Cardboard Prison, The Zig. That's just for human enemies. Robots explode, rock monsters crumble, and spirits are banished.
Word Of God has stated that it's up to players what happens to the Mooks — they could be killed or not, depending on how players roleplay. Named enemies are usually explicitly captured. Robots and rock monsters are confirmed to be non-sentient, the former having no real intelligence and the latter just being the fingers of a massive and powerful ball of jello. The various banished spirits are truly and completely immortal, so banishing doesn't kill them.
Used occasionally in Fire Emblem: Path of Radiance, funnily enough. In chapter 15, you get bonus points for not killing any enemies except the boss (except you don't really kill him), and in chapter 22 you get bonus points and a gift for not killing any priests.
Maintaining the same belief in the comics, Batman in Batman Arkham Asylum never kills. According to his detective mode, his enemies always wind up unconscious. Yes, even the ones who have been punched in the face, or had a wall they were standing in front of blown up. Unconscious, every one.
The game has many ways of preventing you from killing enemies, bordering on The Dev Team Thinks of Everything territory. Knock a guy off a tower, and Batman automatically attaches a cable to his foot. Throw a Mook down a bottomless pit and you hear a splash right away, implying that there's water just out of sight. There's even an invisible wall around the pool of electrified water, so you can't throw anyone in (Batman can still fall in himself, though).
The sequel Batman Arkham City extends this selective invisible wall to all of the many rooftops Batman fights on. Pay no attention to the fact that he's beating people into immobility, and leaving them lying around unable to defend themselves in a city filled with psychopaths, while they're wearing light clothing in the middle of winter.
In Famous has an interesting way of handling this. Killing your enemies in a fight doesn't affect your Karma Meter, but killing enemies who are already bound is marked as an "Execution," which gets you bad karma.
Sort of. As you gain Good Karma, your default attacks get replaced with "stun" weapons. Obviously you are still probably killing a lot of them so it is sort of true, but it rapidly falls into the category of Mercy Bullets and the like. Also, Bad Karma lets you get more Bad Karma as you kill enemies in combat.
In the Thief series, higher difficulty levels prohibit players from killing the guards, presumably not out of morality but for the sake of stealth and forcing them to rely on other means of defeating or evading them.
This could also be for the idea behind Thief that a true infiltrator would not leave any means to suggest they'd been there. A trail of corpses would most likely be noticed, no matter how stupid the guards may seem.
In the first level of Thief 2: The Metal Age, Garrett is not allowed to kill any guards on any difficulty level. This is because the target he is to help escape would be distraught if anyone were to be killed over her.
In Ryu Ga Gotoku, no matter how much of a criminal the main characters are. They do not have the murderous impulses of their Crime Sandbox brethren. It is often used to separate the honorable and dishonorable characters. For Kiryu, his reason for not killing was after the deaths of the only 3 people he considered family, and his vow to renew his life by being Haruka's father-figure.
Carol's response in the titular Carol Reed Mysteries when you try to shoot her attacker at the end of Amber's Blood is a shocked "No, I'm not going to shoot anyone!"
In Sluggy Freelance, Torg made Oasis swear one of these vows. She sorta forgets it for a while and becomes an All Crimes Are Equal vigilante. When she remembers, she cries, "I've broken my promise! There can be no wedding! Why does love bring me nothing but pain?" Ironically, Torg himself doesn't really subscribe to this philosophy, as he was perfectly willing to go in guns blazing and swords swinging during "The Stormbreaker Saga" and "Dangerous Days" arcs.
This trope was actively enforced in the Global Guardians PBEM Universe. Player characters who were casual killers were absolutely not welcome, and those that became it later were booted from the game. Accidents still happened, but for the most part the idea the various campaigns operated under was that real heroes didn't kill criminals. The single exception was the Big Easy campaign, but as that campaign was based on The Dark Age of Comic Books, it got a pass.
In "The Underdwellers", the villain Sewer King uses a small army of abandoned children to steal and commit crime for him, punishing them cruelly when they fail. Batman corners him at the end of the episode and angrily shouts that although he realizes that passing judgment is a matter for the courts, he's sorely tempted to take matters into his own hands.
In "His Silicon Soul", the robot copy of Batman that Hardac created in a final attempt to gain revenge on Batman and Kill All Humans! follows his human template's example all too well. The robot has a Heroic BSOD when it thinks it killed Batman during their fight and sacrifices itself to foil the scheme it had earlier set in motion when it realizes more people will die because of it.
In Batman Beyond, Terry seems to have an attitude somewhat similar to the Batman Begins version of Batman: the series makes it a specific point that he won't kill in cold blood, and he generally tries to make sure his villains rot in jail, but he often won't go very far out of his way to save them, either. He's also consistently willing to use lethal force in the heat of combat, usually in the form of combat pragmatism such as chucking handy barrels of toxic waste.
Joker: "Oh Batman, if you had the guts for that kind of fun you would've done it years ago. I, on the other hand..." (proceeds to attack)
In Justice League, an alternate universe episode sees the Flash die by Lex Luthor's hand, to which Superman responds by killing his archvillain in a gruesome fashion. These events eventually draw the default universe's Lex Luthor to try to ruin Superman by goading him into the same murderous rage. Late in this arc, the Flash appears to sacrifice himself to stop Lex's grandest scheme, to which Lex defiantly gloats. Superman hoists Luthor in front of his face and bitterly growls, "I'm not the Superman who killed Lex Luthor. Right now, I wish to heaven I were, but I'm not."
The prime universe Superman made an exception for Darkseid in "Twilight". After Darkseid's latest gambit to conquer the universe, Superman has had it with the tyrant and stays behind on the exploding asteroid so he can kill Darkseid with his bare hands. The only reason he doesn't manage it is because Batman pulls him and Orion into a Boom Tube to save them. As it stands, Superman does manage to kill Darkseid by trapping him on the self destructing asteroid. It even sticks for four whole seasons. Notably, he spared Darkseid the first time he beat him, and this is when Darkseid had nearly (indirectly) killed Supergirl. Kara herself persuaded him from killing Darkseid that time though.
In the series finale, Superman subtly expresses his hopes that Darkseid and Lex Luthor are dead for good, without his having to kill them. He is so hopeful that five of the other founding seven have to convince him otherwise. According to Word Of God, Superman was actually right this time. Darkseid and Luthor both became part of the Source Wall.
He-Man, in the 80's, seemed to live and die by a code of not killing anyone (which explains why the same villains keep coming back again and again). This causes a crisis of conscience in one episode where he believes he has allowed someone to be killed but it was actually a trick by Skeletor to make him give up his powers.
Aang from Avatar The Last Airbender certainly follows this. However, it appears that his friends are fine with killing people in self-defense, or at least Sokka and Zuko, which he doesn't seem to have a problem with. Sokka killed a company of Fire Nation soldiers in "The Northern Air Temple"!
Word Of God says that none of his past lives actually advocated killing Ozai. It's true. They emphasized the importance of the world's safety and the follies of neglecting his duty and one even says that he should be willing to kill Ozai, but nowhere does any one of them actually say that it is the correct course of action. Aang infers that from what they say, which is representative of his own inner conflict. After all, he's talking to himself.
Meanwhile, Sokka and Toph kill like 500 guys with a massive aerial explosion.
Aang is Reckless Pacifist. The rest of the Gaang seems to advert this trope. It's not shown on srceen, but it would be hard for some of their actions to end without deaths of Mooks. They are Child Soldiersskilled in combat fighting in a war after all.
Aang does admit that he's killed a lot of fire nation mooks before in self-defense. He just doesn't want to purposely kill anyone.
Subtly averted in the Sequel SeriesThe Legend Of Korra. At one point, Korra attempts to attack an apparently defenseless waterbender with two powerful fireblasts. The dialogue and circumstances surrounding the fight imply very heavily that she really meant to kill her opponent; he had spent the better part of the episode hitting her Berserk Buttons, and picking a fight really seemed to be the last straw with her.
Gargoyles has an interesting relationship to this trope. In the modern day, the Clan is generally averse to killing. In the flashbacks to Scotland, though, they don't seem to have any problem with it. At one point, Goliath spells it out that killing someone in the heat of battle was alright. Just attacking someone with the intent to kill, however, was murder.
Averted near the end of the series where a family hunting Demona nearly kill Goliath's daughter. He declares that he will "hunt them down. And I will kill them." He doesn't (initially) change his mind either; the next time he sees them, he tries to kill them by hurling them into a wall of electrical equipment. They only survived because they had special armor on that absorbed the damage. Apparently, the writers had to fight tooth and nail to let that line stay in as it was.
It's even more complicated. From a cultural standpoint, revenge is an acceptable response within Gargoyle society, to the point of it being honorable (at least for the Scottish Clan). Probably for the sake of family-friendliness, one of the first lessons the Gargoyles seem to absorb is that the modern day justice system is now the proper outlet for punishing transgressors. But as mentioned above, that isn't always good enough for the heroes.
In Teen Titans: Trouble in Tokyo Robin gets in some trouble with the law when it looks like he killed the supervillain he was fighting. In the series itself, however, the episode "Aftershock" averts this trope. While the other Titans were holding back, Raven's words and actions indicate she was genuinely trying to kill Terra when they fought. Later Terra decides to pull a Heel Face Turn and stops working for the villain Slade; she accomplishes this by throwing Slade into a pit of lava.
Although killing is rarely touched upon in the show, Codename Kids Next Door seems to somewhat demonstrate the KND, and some villians going by this trope. They instead try to subdue each other as a means to win fights; respectively, the KND would subdue and apprehend villains to imprison them in Arctic base, while the non-killing villians merely do whatever they have in mind with KND Operatives once they overpower them.
A Robot Chicken skit had Batman managing to get past his code by giving the court a testimony that ends with The Joker getting the death penalty.
Saint Jeanne d'Arc took this very seriously, despite being called to lead the French army to victory (from the front!). As she herself stated at her trial, "I have told you often enough, that I have done nothing but by the command of God. It was I, myself who bore this banner, when I attacked the enemy, to save killing any one, for I have never killed any one."
Just like his counterpart in Goodfellas, the real life Henry Hill (who was a big time gangster) claims to have killed no one.
One of the Ten Commandments is usually written as Thou Shalt Not Kill, though experts disagree on the translation and interpretaion. Some people who follow the Bible, and the Commandments, find justifications in extreme stituations, as with Christians who go to war, or Jews who fought back against Nazis.
Jesus commanded Peter to put his sword away, and told him that those who live by it, also die by it. This could be a New Testament account of Jesus' statement on pacifism, especially killing.
The related Hinduism has similar beliefs, mostly grounded in the concept of reincarnation (as potentially any other animal), so it's thought that an animal you encounter could be a reincarnated ancestor.
In general, police forces in most civilized countries prioritize bringing people in alive as opposed to killing them. Sometimes, this doesn't happen. Most cops never even fire their weapon in anger, much less kill anyone.