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.
Opinions differ on how this applies to sentient life other than humans. In general, it's still up to villains to kill other villains when pulling off a Villainous Rescue. 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 heroes to get their 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: the rationale is that if a hero, say, Superman were to kill a bad guy in one story, why wouldn't he simply resolve all of his problems by, for example, incinerating Lex Luthor with his heat vision on sight? Related is Pacifism Backfire, where their reluctance to fight (or to kill as in this trope) may cause Joker Immunity. 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. It's also somewhat common for both stand alone and serial storytelling to feature a character who begins adhering to this trope, but over the course of various dramatic devices, such as a Trauma Conga Line, is finally forced to or chooses to cross the line.
With superhero characters, attitudes toward no-kill policies range from utterly ignoring it (such as the Main Characters of Watchmen), to treating it as a preferred outcome (such as Superman), to strict adherence to it in all cases (such as Batman). 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.
If this trope is used poorly, it can risk leading to audiences seeing heroes holding onto this code as too naive or stubborn due to not accepting that there are villains too dangerous to be left alive and possibly lead to Anti Heroes in the story more willing to get their hands dirty looking more heroic than heroes who swear by non-lethal force.
"Thou shalt not kill" is derived from the sixth of the Bible's Ten Commandments, and the religious implications of taking life in apparent violation of this commandment (which is often translated as "Thou shalt not murder," which results in debates over semantics) are sometimes also invoked in storytelling.
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, and Restrained Revenge. Often goes hand in hand with Doesn't Like Guns, because of the lethal connotations that firearms carry. For a similar trope in video games, see Pacifist Run.
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- The Lone Ranger, in some ways a precursor to Vash, used silver bullets as a symbol of his pledge never to take human life.
- In George Carlin's routine on The Ten Commandments, he ends with this one and claims that because more people have been killed in the name of God than for any other reason, it really comes down to who's doing the killing and who's getting killed. He decides it should be renamed "Thou shalt try really hard not to kill anyone, unless they pray to a different invisible man from the one you pray to."
- 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.
- "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.
- In the Old World of Darkness, Lifesaver is a 3 point flaw that makes you unwilling to take life. Pacifist is a 5 point flaw and is taken literally - the character can do no physical harm to others.
- Defied in Sentinels of the Multiverse. In fact, the resident Batman Expy has a Flavor Text quote warning against the perils of mercy that originally comes from one of the Caped Crusader's villains. Taken in context, this appears to be a proportionate response thing, since most of the game's villains are potentially apocalyptic threats and there are examples in the wider lore of heroes limiting themselves (the example given is that everyone's happy to have Fanatic on their side against villains, but nobody's in a hurry to let her deal with a bank robbery).
- BIONICLE: The Toa take this rule very seriously. One minor character was Orde, a Toa of Iron, who killed a Makuta to save his village, and suffered guilt for years even though...well, it was a Makuta. While What Measure Is a Non-Human? comes into play more often than not, with villains employing robots, Animated Armor and mind-controlled wildlife, none of which this rule apparently applies to, a Toa will only kill a sentient opponent when given no other alternative.
Vakama: Toa aren't killers. If we were, we would have started with you.
- There are a few exceptions, however: Helryx and Tuyet avert this, the former due to leading the Order of Mata Nui where killing is necessary and the latter due to being outright evil. Toa Ignika would have averted this if not for the Toa Nuva due to literally being a few hours old.
- Girl Genius: The leader of the conspiracy in the science dome in England makes the mistaken assumption that as a hero Tarvek holding a gun to him and his compatriot is just for intimidation and he won't actually kill them. Tarvek proves him wrong near instantaneously.
Tarvek: I've never really considered myself the "hero" type.
- 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.
- In The Adventures of Dr. McNinja Mongo the superninja has learned the preciousness of life. And also that fire bad.
- Demon Fist:
Duncan: Your crew fought off all my men without killing any of them. Criminals would not have wasted the effort not to kill their attackers. I can't in good conscience take you all in simply for defending yourselves.
- The Demon Fist almost never kills humans or peaceful demons, even (especially!) Mooks.
- Neither does the Hookshot crew. This pays off for them later.
- Erika and the Princes in Distress : Averted. After attacking Glucose, Erika is actually surprised that he survived the hit. She later gets so furious at him that she starts strangling him with the clear intent of killing him, only stopping herself at the last second when realizing Pita wouldn't approve.
- Decoy Octopus of The Last Days of FOXHOUND passes The Sorrow's test because he has never killed anyone and thus has no one to face. The Sorrow is very surprised and Octopus just shrugs, claiming he's more suited for espionage than fighting.
- Schlock Mercenary: Petey avoids killing if at all possible, and at one point refuses to let a spy go back home because she'll just get needlessly mind-ripped. After the oafans give everyone immortality, he starts going to truly absurd lengths to avoid killing, like teraporting entire fleets that are in the middle of a massive fight. As he says, he can't be sure these people will still be his enemies in hundreds or thousands of years, so killing anyone is like killing future allies.
- Shortly after 9/11, The Onion reported that God held a press conference to remind everyone exactly what He meant by "Thou Shalt Not Kill."
- Averted with extreme prejudice in the Whateley Universe. It's a sad commentary on a superheroic 'verse when the person best known for having a code against killing is a supervillain (Mimeo, a very high-end Power Mimic, so it is not just out of good intentions - he wants to be able to keep getting the power-up from fighting superheroes so he can go after his real targets, meaning that killing his 'donors' would be counterproductive; also, he grew up around mobsters, and was disturbed by the psychological effects becoming a 'made man' had on people he knew).
- All but two of the members of Team Kimba (who are high school students) have been responsible for multiple deaths, and Jade in particular has a body count around 100 - most of them over Christmas vacation in their freshman year.
- A number of characters - including Carmilla, Diamondback, Scald-Crow, Elle Ruud, and Glow - killed during their origin stories. Sandra, Grainne, and Elle were each acting in self-defense, as was Carmilla for the most part (the exception being a Mercy Killing of a fatally injured child she had promised to protect), but Glow's instance was a bit more ambiguous and, as it happens, Ironworks wasn't actually killed when the armored head was cut off - she was actually inside the chest of the armor, controlling it by ferrokinesis, and feigned 'his' death as a way of escaping the losing fight.
- Several of the students, including Carmilla, Tennyo, Bladedancer, Razorback, and Screech, were forced to kill some of the attacking Chessmen and Tiger Guards during the Halloween Invasion of 2006. The trauma of this (and the attack in general) would reverberate across the school for months.
- Jimmy T. was in part traumatized by how hard it was to avoid killing; not only did he destroy one Drop Ship outright while in his Godzilla-esque form (presumably killing those in it), he ate a second one when he used his Blob Monster form, and while he avoided digesting (!) the occupants (leaving them naked and in shock, but alive), he later mentioned that it took a lot of willpower not to.
- Not everyone took it so hard, though. Some of the Ultraviolents even captured a fleeing dropship in secret and forced the mercenaries who had been on it to fight in a hidden gladiatorial arena they'd made.
- It has been mentioned that in addition to several deadly encounters with Nazi spies during WWII, a number of villains have died fighting Lady Astarte due to accidents; like most superheroes, she doesn't choose to kill (and it is a major Character Development moment that she would actually try to kill the Ax-Crazy Blood Knight Deathlist during his attack on the school), but supervillainy is a dangerous field at the best of times, and even in cases where she could do something, she sees herself as being under no obligation to save them from their own mistakes if it would put others at risk.
- In addition, many supposed superheroes and costumed vigilantes show little compunction about killing. The Dark Avenger and the Lamplighter are two of the better known examples, but far from the only ones. This isn't even considering 'heroes' like Jack Rabbit or Iron Mike, who are basically thugs and thieves pretending to be heroes.
- Heroes Save the World: Austin Smith is trying to adhere to this as much as he can. It's disquieting for him when he learns that he has the greatest potential for mass destruction out of the Children located thus far.
- The Red Panda and Flying Squirrel of Red Panda Adventures go out of their way to avoid using lethal force. Though the Red Panda owns a katana, for example, he won't take it into battle so as to not even have the option. They fight with fists, gadgets, and hypnosis. They aren't as firmly wedded to it as other superheroes with a "no killing" code, however. They can and will employ lethal force if the situation calls for it. Fighting non-living foes such as Professor Zombie's undead minions naturally falls into this, but the pair have also been willing to kill if the entire city, or even world is at stake, such as killing the Nazi Ubermensch, Tevas, to keep him away from the Normandy invasion. The one thing that will make either the Red Panda and Flying Squirrel outright abandon this edict is if a villain seems to have killed one or the other. A developing Villain Team-Up decides against killing the Squirrel to get to the Red Panda precisely because they know it will.