Vash, the Martial Pacifist hero of Trigun, goes through the tortures of the damned rather than kill anyone specifically to avoid this trope. Accordingly, the entire last half of Trigun has the Big Bad sending his his suicidal followers to try and forcefully invoke it. Despite the last of them finally succeeding in his mission, it is, in the end, averted.
In Death Note, Ryuk tells Light that if he kills all the bad people in the world, then he himself will be the only bad person left. Light deflects this entirely, because his perspective is Godawful.
It can sound like irony just there, until you get to know him better.
Detective Conan: In the climax of the first season episode "The False Kidnapping and Murder Case," Conan uses this trope to convince Akiko to let Mr. Takei live even though he financially ruined her family and drove her father to kill himself, her mother, and her little brother. As Conan points out, Mr. Takei may be a greedy bastard, but he's still the only family his daughter Naoko (the subject of the episode's titular case) has, and if Akiko kills him, she'll be doing the same thing to Naoko that he did to her. Realizing this, Akiko is moved to tears and thanks Conan for stopping her.
Suzaku in Code Geass comes within an inch of using drugs to interrogate Kallen. He only stops because his final words to her, "You will follow my orders," is the same thing (or so he rationalizes) as what Lelouch does with Geass. Also, but this may or may not count, V.V. tells Lelouch that he's becoming more and more like the Emperor, who he's out to destroy in the first place.
Saji Crossroad's justification for not pulling the trigger on Setsuna F Seiei in Mobile Suit Gundam 00, although it remains to be seen how long he can hold on to his convictions.
Likewise, in conjunction with Marina's song, Setsuna's unstated justification for not killing Ali. The latter doesn't fight again until the penultimate episode.
This appears to be an extremely jarring false dilemma, given that Ali initiated the encounter; killing a villain in self-defense does not equate to burning a country to the ground For the Evulz as Ali did a few episodes earlier. Couldn't it be argued that letting Ali live, or even just escape, is recklessly endangering the lives of practically everyone Ali meets from that day forward?
Actually, this is resolved towards the end. Lyle!Lockon spares Ali when confronting him... but the moment Ali threatens him after this, Lyle shoots him dead for not taking his Last-Second Chance. And this is seen as the right thing to do: Ali spurns his last chance, he's a goner and he cannot harm anybody else. And the one dealing him this is someone who has openly refused to go in the Revenge path, no matter what he thinks and feels.
Louise is warned that her desire to kill Nena Trinity to avenge her family's death will make her no better. Indeed, when she actually DOES kill Nena, she becomes just as psychotic as she was. Thanks to The Power of Love in the final episode, she manages to get better though.
In Fullmetal Alchemist, several characters attempt to keep Roy from killing Envy. At least Riza has no qualms about executing him and offers to shoot Envy herself, but they all agree on the detail that subjecting him to horrible torture first is taking it a step too far.
Scar adds an extra spin by pointing out that if Roy goes through with it, he'll be just like him.
Note that the reason for this isn't because he was going to horribly torture him, since at that point it would be near impossible to hurt Envy without killing him anyway, but because of the reasons for doing so. Roy was going to kill him for personal reasons like vengeance instead of duty, and since his goal was to become leader of the country, he simply can't afford to become the sort of person ruled by his emotions, and specially by bad ones like anger and revenge. If he does, the horrifying mental toll this whole deal is taking on Roy will send him straight into the He Who Fights Monsters path.
In the 2003 anime version, Ed spends every fight with Scar telling him that the slaughter of alchemists and Amestrian troops isn't the answer, and that he must be brought to justice for those crimes as well as killing Tucker's daughter. Each time, Scar shows Ed more and more horrible deeds he's avenging, which Ed always brushes aside. Eventually, upon confronting Scar in Lior when Scar's about to secretly evacuate the city of civilians, lure in the Amestrian army, and kill all of them to make the Philosopher's Stone, Scar pounds into Ed that Lior was purposely sabotaged by Amestris, Amstrian troops have been brutalizing the city, alchemists are killing civilians in the streets right now, and the girl Ed thought he saved in his previous visit (standing right in front of him, unable to speak due to mental trauma) was raped and used as a tool to create even more violence, and that all of it was partly Ed's fault. Ed still refuses to condone what Scar is doing, but when Scar orders him to get out of Dodge with the refugees, Ed shuts up and meekly complies.
In the second season finale of Rozen Maiden, Jun utters the line when Shinku was about to kill Barasuishou. Bad move. She uses the chance to defeat Shinku.
The titular character in Ginga Densetsu Weed. So much, especially during the final episode in which Weed has the chance to finally avenge his fallen comrades by killing Hougen once and for all. His father intervenes and is about to kill him when Weed pushes him out of the way, claiming that his father would be no better than Hougen if he'd killed him. There's quite a few examples in this series, suffice to say.
In the Full Metal Panic! novels, this is pretty much what Tessa tells Sousuke during the Behemoth arc, when he decides that shooting Takuma would be an effective action to take. "It would be the most logical and secure route, but we can't go about it that way," she said as though trying to convince herself. "If we were to kill him, we would be no better than them. Our organization would lose all meaning." Sousuke is slightly skeptical about this (seeing how Kalinin, his adoptive father whom he was always taking orders from, undoubtedly would have done what he was about to do), but nevertheless follows orders, seeing how he doesn't really care either way. Of course, later on, the fact that she let Takuma live led to ahugeamount of destruction...
In Kara no Kyoukai, Mikiya doesn't want Shiki to kill Lio, despite him being a crazy cannibalistic superman, because he feels that Shiki isn't and shouldn't become a murderer. At first she defers to his wishes. Then Lio stabs Mikiya in the eye, seemingly killing him. An enraged Shiki then kills him anyway. Mikiya, after he wakes up, is somewhat annoyed, but seeing that Shiki's murderous side remains under control, says that he'll get over it.
When Vegeta is finally taken down by Goku and his True Companions, he tries to crawl back to his space pod and make his escape. Krillin grabs a nearby sword, intending to put the heavily wounded Vegeta down once and for all. But Goku stops Krillin and asks him to let Vegeta live. In some versions of the English dub, he justifies this by telling Krillin that killing Vegeta in cold blood will make them just as bad as he is. The original story averts this, however: Goku doesn't care about the morals of the situation, he only wants Vegeta to live so he can fight him again, even though he knows this potentially puts the Earth at tremendous risk. Goku even admits this is a selfish request.
Goku decides not to finish off Frieza after defeating him in battle, which is an extremely hard decision for him to make. He even gives him some of his energy.
In New Grappler Baki, Katsumi Orochi gets beaten the hell out of him by an escaped mass murderer named Dorian. When Retsu Kaio returns the returns a beating to him, Katsumi enters the scene and soaks Dorian with gasoline and is about to torch him, when he suddenly stops and says that if set him on fire, he would cease to be a true Karateka, so he walks away. Then he goes: "Actually, that wouldn't bother me at all." Cue the BBQ/delicious subversion.
In the Story Within a Story of the music video arc of Skip Beat!, Kyoko's character, an angel, is forced to murder Sho's character, a demon, in order to protect her friend. This fills Kyoko's character with such guilt and rage that she becomes like a demon herself.
This is found a lot in Rave Master the reason Haru gives for not wanting to kill his enemies is because he'll be just as bad as them. He didn't actually kill anyone besides Doryo.
Turns up in the climax of the final episode of Noir. Not that that stops the protagonists from killing off Altena anyway.
Used in Omamori Himari. Made especially justified by the fact that the girl that Shizuku wanted to kill wasn't the man who killed her family. The girl was the great-granddaughter of the man who genocided the Mizuki race, and happened to be completely ignorant of her long-deceased ancestor's crimes.
Subverted in Digimon Savers. Marcus and Shine Greymon have defeated Kurata's One-Winged Angel form, and effectively have him completely beaten and begging for mercy. Not a single one of Marcus' allies pull this trope on him, instead actually encouraging him to kill him! What avoids making this a Start of Darkness is that Kurata is really that much of an evil bastard
Deconstructed in Adventure 02. Evil Digimon are released into the human world and Yolei and Cody are horrified when their Digimon are forced to kill them. It's quickly pointed out that the evil Digimon had no problems about killing innocent people and they would have killed Yolei and Cody had their Digimon not saved them. They tried to send them back to the Digital World but no other options were available.
Hugely averted in The Astounding Wolf Man where Gary sets out to kill Zechariah for what the latter did to his life. When Gary finally gets the chance he takes it.
Batman seems to find himself in this predicament fairly often:
This is invoked maybe half the times he goes up against the Joker. The comic book series also highlights the cost of Batman's decision: Joker's victims are estimated in the thousands, all of whom Batman is indirectly responsible for.
In Knightfall, Dick Grayson (Nightwing), and Tim Drake (Robin), come upon Bruce Wayne just as he has apparently killed the mook who was attacking him. An outraged Nightwing declares: "killing this creep doesn't make you as bad as the slime we used to fight, it makes you worse, because THEY never stood for anything!" Of course, it turns out Bruce had used a move that makes the victim appear to be dead for a little while..
In Under the Red Hood, Batman openly admits that he fantasizes about killing the Joker every single day, but won't because he believes that if he starts killing, he won't be able to stop. Used to justify why he hasn't taken out the Joker at the very least, given how the latter is utterly beyond redemption and has racked up a ridiculously substantial body count. But if it's okay for Batman to kill someone, it might be okay for Batman to kill anyone.
In Death Of The Family, the Joker pokes fun at this idea. Even he believes that Batman probably could kill the Joker without becoming anything like him. He taunts Batman and asks him what is the real reason Batman won't kill him. It's because Batman has a similar "delusion" as the Joker that his life follows a theory of narrative causality. Killing the Joker would simply force Gotham to send someone worse to challenge Batman.
Subverted in Birds of Prey #73, when Vixen thanks Huntress for "not giving in" and executing evil cult-leader Sovereign Brusaw, to which Huntress replies "Don't thank me... The truth is, my crossbow jammed."
Played straight at the end of Batman/Huntress: Cry for Blood, when The Question begs Huntress not to kill Santo Casamento; she does it anyway.
Kremlin references the trope in Ex Machina Special #2 and states how he thinks such is an out of date, fairy tale ideal.
Averted/Played with in Atomic Robo when A now elderly Skorzeny informs him that he was the one that killed Nikola Tesla, Robos creator, during WWII, and he did it to steal the man's inventions to use against Robo, who was at the time serving in the US Army. Robo picks up a gun, aims... and then puts the gun away, informs the Nazi that he knows he's dying of cancer, and that he won't be dying like a soldier, instead dying alone, in a hospital bed, in agony.
Superman's long lived in fear of sliding down this slippery slope, and, in the Silver Age Alternate Continuity story Whatever Happened to The Man of Tomorrow?, it comes to pass when Superman, in order to save himself, his remaining friends, the world and possibly the entire universe, is forced to deliberately kill the villainous Mr. Mxyzptlk. While everyone else tries to convince him that his decision was clearly justified, he refuses to take the risk, cites this very reasoning as a condemnation of his actions, and so carries out a promise that if he should ever take a life, it would be the end of Superman. As it turns out, he fulfills his promise by stripping himself of his powers and living the rest of his life as an anonymous human, which actually gives him a happy ending after all.
Averted in one post-Crisis arc when Superman is brought to an alternate universe where three Kryptonians, each more powerful than he is, released from the Phantom Zone end up killing all life on Earth. When they promise to find their way to Superman's universe and do the same thing, he feels he has no choice except, as the representative of justice on the planet, to execute them through kryptonite exposure. When he gets back home the next story arc has him suffering a Heroic BSOD as he deals with the choice he made.
Spider-Man does it often. For example: he uses that argument while arguing with Clint Barton and the rest of the New Avengers that thinks about killing Norman Osborn, and not so long ago stopped Harry Osborn from killing his old man by saying, that if he'll do it, he will become exactly that kind of man his father always wanted him to be.
Peter got one at the end of the Grim Hunt story, where he's shown a vision of the future if he kills the just-resurrected Kraven the Hunter. The reaction of the fandom was... unexpected, as the two page spread that encompassed the vision not only showed Peter apparently becoming a complete and utter Bad Ass, but featured the fan-favorite "Happy Birthday" costume. Cue the fans shouting "Kill the guy!" at their comic books.
In Spider-Man: Noir, this gets played interestingly because Spidey gets this lecture from Aunt May (who doesn't know his true identity) after he's already killed the Vulture - even though he was going to kill and eat her and she knew he did the same to her husband! During the lecture, Peter thinks to himself that she doesn't understand... but after she says she doesn't want to live in a world where men kill each other like animals, he realizes he's the one who didn't understand. He tries to live up to her expectations when he raids the Goblin's torture house, where he doesn't kill any of the Goblin's men except Kraven, and only in self-defense. He even refuses to kill the Goblin when given the chance, because he wants to see him properly brought to justice.
Wolverine regularly tells younger, softer, less inclined characters to back off or stay outside before going in to finish off the bad guys. Normally, he states in some manner that only people who are going to do "what needs to be done" should go in. This is actually one of the reasons Wolverine is even included on some of the teams he's been on. For example during the Infinity Gauntlet affair Nick Fury explicitly states this is the reason for his inclusion "...because when it comes down to it, none of you are killers. He is." The same reason applies for why Fury brought the Hulk in as well.
Played straight in one issue of Wolverine's self-titled comic, when confronting Reno and Molokai, hitmen who had murdered Jubilee's parents.
Wolverine: Heck, they killed your parents. It's a good killin', ain't it? Ain't it?
Jubilee: You kill people! You've killed so many, and...
Wolverine: Yeah. You wanna sit up some night with me and talk to all of 'em?
In a later issue of Generation X, Jubilee learns that the grandfather of one of her classmates ordered the killing of her parents, and goes after him. The rest of the team go after her, prepared to cite this trope at her, when they discover that she never intended to kill the guy.
Synch: We were afraid that, hanging with Wolverine, you would have wanted to...
Jubilee: You guys don't get it, do you? It's because of Wolverine that I won't kill.
Everyone's favorite many angled one, Shuma-Gorath, is immortal for this very reason — anyone who kills him will start to become a new Shuma-Gorath (unless they have a similar level of immortality). Doctor Strange took his own life rather than allow that to happen, which still didn't work. Fortunately an ally was able to purge Shuma's energies from Strange and bring him back.
Lampshaded during Planet Hulk, when Miek kills the Imperial Headman who murdered his father in front of him, in view of the man's own children. The Headman notes that Miek is now just like him.
The subject doesn't come up much in Sin City, although the series makes no qualms about presenting its Antiheroes as batshit insane in some cases.
In Denny O'Neill's The Question, Vic Sage finally takes down the first arc's Big Bad, the corrupt Reverend Hatch, and explicitly invokes this Trope. "I could kill you... but then I'd be something just like you. Something vile. So I won't." He turns his back and walks away... Hatch draws a concealed weapon... and then Myra shoots Hatch in the back. "Maybe he wouldn't... but I will."
Played with in issue #20 of IDW's Transformers: More than Meets the Eye. Ratchet tries to give this speech to First Aid when the latter wants to go track down and kill one of the arc's villains, but the latter ain't buying it.
Ratchet:No. If we kill him, we're no better than him. If we kill him, he wins. First Aid: Yeah, except—we are better than him and he doesn't win. He doesn't anything. He's dead. That's the point.
In With Strings Attached, John uses this, more or less, to calm Paul down after the Hunter slaughters a bunch of wolves in front of him and Paul struggles not to go berserk on his ass. It works on both Paul and the Hunter; the latter is stunned that people with such power would choose to not use it to kill, and he eventually does a Heel-Face Turn because of it.
Scar: What are you going to do? You wouldn't kill your old uncle, would you?
Simba: No, Scar. I'm not like you.
He immediately followed this with offering him a particularly ironic form of Cruel Mercy.
In Batman: Under the Red Hood, Batman explains that the reason he didn't kill the Joker after the latter killed Jason Todd was that if he went there, he'd never come back. Jason failed to understand the reason behind his statement, though
Films — Live-Action
In Return of the Jedi Luke refused to kill Vader, because he realised that doing so would make him no better than Vader was. This is a more literal case than most, as if Luke couldn't let go of his emotions, and if killed Vader, he would do it in anger, cementing his path to the dark side and becoming the new Sith Lord.
Played straight in the gag-worthy ending to Knight Rider 2000.
Played straight, then subverted in Rush Hour 2. Lee has Ricky Tan, the Big Bad at gunpoint, when Ricky starts taunting him, and Lee's partner Carter tells him, "He's trying to trick you, don't go too far." That is until Ricky says Lee's father was "pathetic," at which point Carter changes his tune to "Shoot his ass!"
While it isn't a villain, this trope is basically what stops Sarah Conner from killing Miles Dyson. She sees in that moment that if he kills him, she'll be just like the other Terminator who tried to kill her to change the future.
Played straight in the finale of the film Prince Caspian. Caspian decides at the last moment not to kill Miraz in an arranged duel, even though he's fully prepared to hack-and-slash his way through Miraz's army a few moments later, had no trouble chopping up bad guys at the castle a few nights before, and could probably fill a book with unassailable reasons that necessitate Miraz's death.
Played dead straight in X-Men Origins: Wolverine, by Logan's squeeze Kayla, in regards to him killing off Sabretooth and/or Stryker. Two men, who have been rounding up mutants like cattle, are effectively above the law, and will most likely continue to hunt Logan down as long as he lives. All of which could be solved by a little extra stabbing...
Played even straighter in X-Men: First Class, where Erik does kill Shaw and become just like him: after most of the film is spent with Charles and Erik arguing over whether Erik should kill Shaw, Erik catches up with Shaw, who attempts to explain himself. Erik's response is to say 'you're right', kill him anyway, and picks up where he left off as regards the mutant-supremacy agenda.
Subverted in Darkman. The movie ends with the hero dangling the villain off a building. The villain goads Darkman into killing him and then explains why Darkman won't let him fall, asserting that he could never live with himself. Needless to say, down he goes. "I'm learning to live with a lot of things," mumbles Darkman.
Done mind-bogglingly badly in Cave Dwellers. As Ator is about to kill the villain, his old mentor declares that to kill him would be murder, and that "It would make us no better than the barbarians!" (Tom Servo points out, "We are barbarians!") Ator drops his sword, turns around... and is only saved from a Back Stab by his Asian sidekick nailing the villain with a throwing knife.
Joel (as Ator): Then why the heck did I hang-glide in here, anyway? note Don't ask about the hang-glider.
Subverted in Train. When the head of the organ harvesters who've killed most of the protagonists is at the mercy of the Final Girl, she tries this.
Dr. Velislava: You do this, you are exactly like us. Alex: Maybe I am. [proceeds to beat Velislava up and then burn her to death]
Played straight in Mannequin 2: On the Move. The Prince has the Count by the throat, dangling over the edge of a hot-air balloon basket, high above the streets of Philadelphia. He decides that he can't kill the Evil Count. Of course, as soon as lets the Count back in the basket, the Count reminds us that he has no hesitations about killing others.
Invoked in Batman Begins when Bruce Wayne (not yet Batman), refuses to kill a criminal as part of his training
Henri Ducard: Your compassion is a weakness your enemies will not share.
Bruce Wayne: That's why it's so important. It separates us from them.
During the climax of the movies, after Batman defeats Ra's, he doesn't kill him because that would give Ra's the final victory, but finds a loophole. Since they are on a train that's going to crash.....
"I won't kill you, but I don't have to save you."
Both the film and book versions of The Lord of the Rings trilogy have this regarding trying to use the Ring. Film Galadriel puts the most dramatic point on it. Unlike most of the characters who want to claim it, she's probably not deluding herself about her ability to do so, but it all still ends with Middle Earth under the rule of a "dark" overlord.
Suspect Zero: The film's antagonist O'Ryan ultimately wishes to drive FBI Agent Mackelway to kill him, in an effort to force Mackelway down the path to taking over his mission as a Serial-Killer Killer.
This trope is delivered word for word by Morgan Freeman's character in the end of Unleashed.
In the first Lethal Weapon movie, Riggs and Joshua fight on Murtaugh's front lawn. Just as Riggs has Joshua in a killing pose, he stops, saying Joshua isn't worth it. As two uniformed officers are taking Joshua into custody, he grabs a gun and starts taking aim at Riggs. Murtaugh and Riggs both open fire on him, justifiable defense.
In Scanners II: The New Order, this seems to be the reason why David doesn't kill Forrester at the end and to make it clear to the public that scanners are not a threat to them.
In Animorphs, when the war ends, Jake refuses to kill Visser One. Tobias, enraged, demands to know why, claiming that Visser One was the one responsible for the entire war. Jake replies quietly that they "don't kill prisoners."
The Visser immediately mocks his hypocrisy; Jake has just killed seventeen thousand unarmed, helpless Yeerks. Not to mention he and the others had blown up the shopping mall to take out the Yeerk pool beneath it, which killed thousands of unhosted Yeerks and hundreds of innocent people.
Rachel is about to kill Tobias' captor Taylor, but Tobias urges her: "Be Rachel. Not her."
In Harry Potter and the Prisoner of Azkaban, Harry stops Sirius Black and Remus Lupin from killing Peter Pettigrew, who had betrayed Harry's parents' whereabouts to Voldemort, because he was sure his father wouldn't have wanted his old friends to become murderers, even in revenge for his own death. Notably, however, he didn't regard this as any kind of mercy for Pettigrew's sake, and was happy for him to go to Azkaban prison. Sadly, unforeseen circumstances interfere and Pettigrew escapes with ultimately disastrous long-term consequences, though the 'mercy' itself has slightly more positive results.
Used in the book Komarr in the Vorkosigan Saga by Lois McMaster Bujold. Komarran terrorists are about to activate their new weapon, which they think will lead to a bloodless coup, but which will actually blow up the space station it's positioned on, in revenge for the Barrayan massacre of Komarran hostages a generation ago. When the army closes in, they threaten to murder their hostages, Ekaterin Vorsoisson and Aunt Vorthys, if Imperial Auditor Miles Vorkosigan doesn't order the army to back off. Miles tells them, "Please observe that you have now gone as far as you can without turning yourselves into a perfect replica of the enemy you set out to oppose." The terrorists surrender, but partially because Ekaterin had already destroyed their device.
In The Dresden Files universe, the highly addictive nature of black magic means the White Council enforces this for any type of magical killing of a human. So, someone who uses black magic to kill a human, even if that human was a black-magic murderer themselves, will almost always jump off the slippery slope and become a serial murderer themselves.
In Fool Moon there's also a slight aversion: When Tera West hears this line she replies "No I won't. I'll be alive, and he'll be dead."
The Discworld novel Jingo implies that Sam Vimes believes something like this. He's all right with killing someone by accident, but a pre-meditated killing leaves him deeply troubled. In the next City Watch book, Night Watch the Gargoyles parallel is even more explicit, as he's now shown to be perfectly willing to kill other people intentionally when he's in the middle of a pitched battle.
Another Terry Pratchett book, Nation, has this discussed as a theme, though the focus is more on the thinking like the villain, than the actual act of killing. First Mate Cox is cruel and evil, enjoying killing simply for the joy of it. When he mutinied against The Good Captain, said captain nearly fell for this trope. Later, Mau is afraid of thinking like him, but decides that any hunter must think like their prey, yet does not turn into it.
Subverted in Shadow of the Giant, when Bean grabs Volescu, a man who tried to redesign the human race by replacing them with people like Bean: super-smart but doomed to die by age twenty or so, and also is directly responsible for Bean being like that, by the throat and threatens to kill him:
Petra: "Please don't kill him, Bean. Please."
Bean: "Remind me why."
Petra: "Because we're good people."
Volescu: *laughs* "You live by murder. How many people have you both killed? And if we add in all the Buggers you slaughtered out in space..."
Petra: "Ok, go ahead and kill him."
They end up not killing him in the end, if for no other reason than he has information they need.
In book ten of A Series of Unfortunate Events, with Sunny captured by Olaf and one of his top goons heading towards them, the kids decide to set a trap to gain a hostage of their own to possibly trade for their sister. But before they do it, they have an epiphany: if they capture this woman, it will make them evil like Olaf, so they surrender to her and she takes them to Olaf. And these kids are supposed to be the smart ones in a world of useless adults.
Played with in Star Trek: The Genesis Wave. The Bolian colonists on Myrmidon have seen their planet terraformed into an uninhabitable jungle by the Lomarians. When they fight the Lomarians by setting the jungle alight, becoming frenzied and desperate, Mr. Mot sadly reflects that they've sunk to the level of the enemy; single-mindedly destroying an entire environment and those who call it home. However, while he acknowledges that his people are indeed "just like" the Lomarians, and is saddened by it, he doesn't actually condemn them or say they were wrong; at least in this case, becoming "just like him" is seen as a sad necessity.
Drizzt Do'Urden has been in a position of wondering whether to spare his Arch-Enemy Artemis Entreri, considering leaving him alive also as potentially unethical because he's just going to kill more innocents if he lives. Not that it matters much what he decides, as Entreri has Joker Immunity. Drizzt also used to be determined never to kill his fellow drow, because that's what they always did to each other and that was what he didn't want to become like. Once they catch up to him in his new life and start attacking, though, he quickly stops angsting over that because it would be stupid. Basically, he realises it's not this trope when it's self-defence.
In Sarah A. Hoyt's Darkship Thieves, Thena wants to kill her father, both for justice and because he wants to transplant his brain to her body. She fails. Kit points out that it proves being his clone didn't make her like him.
Subverted (and possibly lampshaded) in the Battlestar Galactica episode "Blackmarket." Lee is holding the big bad at gun point, and the big bad claims that Lee will not shoot because "You're not like me." When Lee does not lower the weapon the man starts to say it again and Lee shoots him dead. The producerssay this never happened, and all events from that episode (With the exception of the death of Pegasus's CO) are never referred to again.
24's Jack Bauer doesn't bother with this trope, killing several villains in cold blood including his mentor Christopher Henderson, who no longer has any reason to continue being a bad guy at the time of his death.
Played straight in the final season where on a Roaring Rampage of Revenge Jack slaughters countless members of the current conspiracy-of-the-day in ways that cross even his own personal lines. In one of the last hours, he holds another of them in on it, Jason Pillar, hostage, and with whatever composure he has left Pillar screams that by this point Jack is as much a murderer as they are. The fact that in this rare case he's actually right hurts, and in the end when he sobbingly begs Jack not to shoot him because he wants to see his own daughters again Jack ultimately can't bring himself to kill him.
In an episode of Stargate SG-1, Daniel Jackson and Captain Samantha Carter come across a vat of young Goa'uld symbiotes, Daniel is about to shoot it when Captain Carter says that if he does he will be as bad as the Goa'uld. They begin to walk away but then Daniel suddenly turns and fires at the vat anyway, killing the symbiotes.
Subverted in Stargate Atlantis, Shephard and Michael fight on the roof-tops. Michael hangs from the roof by his finger-nails. Earlier in the episode. Michael had threatened Teyla's baby. Teyla stamps on one hand and then the other. Michael falls to his doom. Mom morality pwns Hollywood morality.
This is part of the philosophy of the Ancients in Stargate SG-1. They believe in the free will of every being and even though they have the power to eliminate every threat in the galaxy, they still don't do it. This, however is taken to such an extreme that one can only declare them guilty.
Then again, later seasons reveal the existence of the Ancients' evil counterparts, the Ori. A rival group of ascended beings who are more than happy to maintain complete control within their galaxy by enslaving everyone. The Ancients were actually exiled by them because of this difference in philosophy, so it makes sense the Ancients themselves would be really worried about slipping down the slope if they start interfering and solving mortals' problems. They do prevent the Ori from acting directly in our galaxy, but the Ori's followers are free-willed (if misled) mortals, so the Ancients don't do anything to stop them.
Faith pulls this on Buffy the Vampire Slayer to break a Mexican Standoff where she and Buffy both have knives to the other's throat. Unique in that Faith seems to somewhat want Buffy to kill her and thus become like her, as some form of revenge. Yeah, Faith had issues...
Xander raises this concern, when Buffy is intent on killing Faith. Not only does she ignore him, after she failed and Faith wakes up from her coma, Buffy follows her onto Angel for another go.
In Issue #8 of Buffy the Vampire Slayer Season Eight, Buffy finally says what we all think of when this trope is done poorly.
Willow: You know what I mean. It's not like we can just stake these grunts in the heart, right? Not killing humans is what separates us from the bad guys.
Buffy: No, not being bad is what separates us from the bad guys.
IN S5's "The Gift" Giles hasa Genre Savvy moment and decides not to expose Buffy to these moral questions with Ben/Glory, and kills Ben/Glory himself.
Although never actually spoken, it is heavily implied (and even nearly subverted) during the scene at the Huntsman's treehouse in The 10th Kingdom, when Wolf is about to kill him with the magic axe and Virginia stops him.
Common in Doctor Who, where the highly moral Doctor often must make a difficult decision between killing his enemies (technically a violation of his principles, though he's forced to do so more often than not) or showing mercy at the risk of them going on to hurt others. He often settles for giving them a fair chance to leave peacefully, even pleading with them to "just walk away." A noteworthy example in "The Doctor's Daughter", when he seems about to kill a man in vengeful anger but then puts the gun down, explaining, "I never would," despite his rage.
The Doctor almost references this trope by name in "Genesis of the Daleks" when he hesitates in killing a large number of baby Daleks, stating that if he did so he'd "become just like them". In the new series episode "Dalek", the Doctor IS prepared to simply blow away the titular creature, but Rose pulls this trope on him.
And yet sometimes his "mercy" is pretty severe, in "The Family of Blood" where one member is put into a field, alive and conscious, as a scarecrow and another is put in the background corner of mirrors - all mirrors
He does, however start to slide down that slippery slope when he attempts to kill the doctor that the Gunslinger is trying to kill for making him in "A Town Called Mercy." Luckily, Amy is there to snap him out of it. To be honest, The TL!Doctor is not a fan of the Alien!doctor who created cyborgs to go to war, killing thousands to bring about peace, though it is hypocritical to judge the Alien!doctor when The TL!Doctor practically committed genocide to end the Time War, making him The Last Timelord.
Given Eleven's massive levels of self-hatred, he is probably aware of that fact.
A variation (and eventual subversion) occurs in the Babylon 5 episode "Deathwalker" when the title character flaunts her miracle cure to Commander Sinclair, a miracle cure that requires the death of another living being to manufacture. "The billions who live forever will be a monument to my work, and the billions who are murdered to buy that immortality will be the continuance of my work. Not like us? You will become us." The subversion comes when the deal is made to research her cure anyway... only to have the Vorlon ambassador destroy her ship, claiming the younger races were not ready for immortality.
In "In the beginning" (a spin-off depicting the Earth-Minbari war) Dr. Franklin is revealed to have gathered extensive biological information on Minbari after he tried to cure a group of sick aliens, but he's unwilling to pass the info to the military, since they'll most certainly use it to create bio-weapons and he considers using it to be as bad as the genocide that Minabari are inflicting.
Also in the season three episode "Dust", Ivanova is about to let loose the full power of Babylon 5's defense grid on the Psi-Cop Alfred Bester only to be stopped by Sheridan. He even incorporates the trope name in the speech (somewhat), telling Ivanova to fight Bester and evil baddies like him without becoming them.
Sheridan later sacrifices dozens of innocent and disabled telepaths to undo the Psi Corps-supported coup. But he spares Bester's girlfriend because he's a good guy.
Subverted and yet played straight in an episode of Fast Lane. Billie has the criminal who ruined her life at gunpoint when one of her male partners arrives and yells, "Don't do it! Not like this! (Dramatic pause) Use my gun! It has a larger caliber so it will be more satisfying." Her other partner has a similar do-it-this-way comment (which I can't remember), but they're both using reverse psychology to convince her not to kill the criminal. Maybe. Well, she doesn't, anyway.
A non-killing example: In an episode of the sitcom Taxi, an arrogant hairdresser (played by Ted Danson of Cheers, of all people) gives Elaine a truly horrific new hairdo. She, Alex, and Louie go back to confront the man, who is utterly unrepentant. An enraged Elaine is about to dump a large bowl of some noxious liquid over the man's head, when Alex gives the standard "no better than him!" speech. Elaine agrees and backs down, only for Louie to gleefully state: "She may be better than you, but I'm not!" BLOOP.
Played bizarrely straight (and by a character who should have known better); an episode of the second season ends with one of the agents pondering, apropos of almost nothing, how much difference there REALLY is between the offenders that they hunt, and the agents themselves. In this corner, an antisocial, sociopathic recidivist murderer who was abused by his parents for fifteen years and slaughters innocent women purely for the sexual thrill that it gives him. In the opposite side, an agent with ten or fifteen years of experience in fieldwork with the FBI who is willing to fire their weapon ONLY in cause of self-defense or the preservation of another life (and even then, only with utmost angst over the decision afterward), and who has dedicated their professional life to the incarceration of those who would commit such heinous crimes. Yeah, that's a real slippery slope right there.
In an episode of season three, a man takes it upon himself to rescue his kidnapped daughter, but is interrupted from killing him by Agent Reid, who tells the man that if he kills the kidnapper, he'll introduce a cycle of violence into his daughter's life. As he pleads with the father, asking him when the violence will stop, the man whispers, "Tomorrow," and shoots the kidnapper in the head.
MacGyver, being a Technical Pacifist, was rather fond of pulling this gem out whenever his sidekick-of-the-week had the villain at their mercy.
Hilariously played with in Firefly, where Mal wins a sword fight and is encouraged to kill his opponent who is beaten and lying on the ground. He's told it's a matter of honor, but Mal responds, "Mercy is the mark of a great man." Then stabs him (slightly).
Mal: "Guess I'm just a good man. [stabs opponent again] Well, I'm all right."
Sort of inverted in an episode of Are You Afraid of the Dark? ("The Tale of Cutter's Treasure"). The main character was fighting a pirate ghost, and was about to finish him off with a dagger...then realized that the ghost was trapped guarding his treasure, and wanted to be at peace. They decided that after all the people he killed in life he didn't deserve it, and did the LESS MERCIFUL thing by LETTING HIM LIVE.
Said word for word in "Fragile" by Clark to convince a second generation meteor freak not to kill her similarly-powered father.
One of the only times Clark has intentionally killed an opponent was when he faced the alien warrior, Titan. Clark reflects that, for a moment, he actually enjoyed it. He also clearly killed Brainiac (or tried to), rationalizing that machines aren't alive.
In a different twist, Chloe tries to convince Clark that Lex is a big threat and Clark can't just let him walk away. Clark asks if this means he must kill and says that it would make him Not So Different.
A postmortem version. Clark says something like this after he finds out that Oliver killed Lex Luthor.
In the 1998 series Merlin, Vortigern kills the Knight Templar King Constant and takes his throne, then promptly turns into an even worse king himself. Merlin comments, "And one tyrant smoothly passed the crown to another, even worse."
In The Shield, Vic Mackay regularly breaks the law to catch criminals. One could argue that he's better than the criminals, because he's breaking the law to keep his precinct safe. But then Cavannaugh develops a vendetta against him, and resorts to breaking the law to bring him down. So... he becomes a corrupt cop to bring down a corrupt cop. One of the most justified examples of "You will be no better than him", since if you think that breaking the law to catch a criminal is acceptable, then Vic should be allowed to do his thing. And if you think that it's not acceptable, then Cavannaugh's actions are not justified.
Justified in the 2006 Robin Hood series: Prince John promised that if Robin kills the Sheriff, then John will kill all the Peasants in Nottingham.
Used and justified in the Farscape three-part episode "Look At The Princess." At the end of the trilogy, Crichton has Scorpius at his mercy and is ready to shove him into a vat of acid; however, at the last minute, he finds himself unable to go through with the murder, and lets Scorpius off with a warning. Later, it's revealed that this had absolutely nothing to do with any fine motives on Crichton's part- he literally couldn't do it because the neurochip Scorpius had implanted in his brain wouldn't let him. Of course, even if he had been able to overcome the chip, it probably wouldn't have done much good, because as soon as Crichton has left the room, Scorpius casually reveals that his gimp suit can't be so easily dissolved by the acid.
This is why Warehouse 13 insists on Tesla stun-guns and bronzing. As Artie puts it, killing "taints your soul."
Done in a comedic effect in Mockingbird Lane. Although Herman doesn't kill Steve (he actually died falling down the hidden staircase,) Grandpa installs Steve's heart to replace Herman's failing one. Herman then takes Steve's place as an Explorer Scout leader, the only type of socializing Steve did after his wife died.
In the first season of Once Upon a Time, flashbacks reveal Snow White willingly lost her memories of Prince Charming and the effect began to darken her personality. Because of this she ultimately decides to kill Regina since she's responsible for her misery. Prince Charming eventually learns that if she succeeds, she'll become as corrupted as Regina and sets out to both stop her and restore her memories of him.
In season 2, Snow actually goes through with it when she tricks Regina into killing her even more evil mother Cora. When Regina pulls out her heart to take revenge, she sees there's now a black mark on it and puts it back, wanting to see Snow continue down this path until she becomes evil herself.
Towards the end of the Karda Nui arc in BIONICLE, a shadow corrupted Takanuva enters a near-Unstoppable Rage and almost kills a few Makuta in revenge for what they did to him. He is stopped when Kopaka pulls a Get a Hold of Yourself, Man! and cites the Toa's Thou Shalt Not Kill code when he tells that he'd just be giving into that darkness inside if he did it.
This is very much true in Christianity. Even in the earliest books of the Old Testament, personally carrying out revenge by death is outright forbidden. In fact, not paying evil unto evil, but rather with forgiveness, is a fundamental aspect of Biblical Christianity.
Lucius the Eternal from Warhammer 40,000literally embodies this trope. As in, if you kill him, you will turn into him. Right down to the clothes, which will only have one difference: your soul, nailed to the surface with all the others who have killed him in past. The only possible way to avoid this horrific fate is to kill him without feeling any kind of pride at having done so...or possibly to take an extremely fast-acting poison as soon as you kill him. Unfortunately most of his victims don't know this.
Birthright: Takes this quite literally, as a hero attempting to slay an awnshegh (blood abomination monster) may end up getting overwhelmed by the evil power in the monster's bloodline and become just as tainted.
Little Sally: Wait a minute! You can't just give her the rope! Hot Blades Harry: Why not?! Little Sally: Because killin' her would make us no better than them. Little Becky Two-Shoes: Haven't you heard, Little Sally? We are no better than them. In fact, we're worse.
In Splinter Cell Conviction, the final boss gives you a choice to either kill him or spare him, either of which nets you an achievement. In the twist ending, if you spare him, Grim kills him instead.
In Saints Row 2, the boss gives you this choice but the player kills him mid-sentence instead.
The 360 game Ninety-Nine Nights has the two main characters in different play throughs in the same situation where they get a chance to kill the main villain. The noble Aspharr spares the goblin king and is bathed in holy light for his mercy. The vengeance driven Inphyy kills him and the holy powers she had been using the entire game reverse on her making her the new villain. The game also features the second corollary of this trope in that in the course of sparing this guy you killed thousands of his soldiers, his priests and various commanders (Not to mention kings of other races) with no such karmic backlash.
Advance Wars: Dual Strike features Von Bolt - an old man using technology to sucking the life force from the land in order to extend his own life - saying this to Jake in his Not So Different speech during the game's ending; if Jake shoots him, they'll both be guilty of the same thing: killing others to save themselves. The game then lets the player decide. If you say yes, Jake destroys the MacGuffin Von Bolt uses to drain the land's life force and dooms him to die of old age; if you refuse, Hawke shoots Von Bolt.
Invoked in Advance Wars: Days of Ruin. Lin (good girl who frequently has Anti-Hero tendencies) is about to kill Greyfield/Sigismundo (evil megalomaniac who borders on A God Am I in his insanity), when he tries to pull off a speech like this trope, saying she'll be just like him if she kills him. However, unlike other examples, this isn't portrayed as a real moral dilemma so much as just Greyfield/Sigismundo's cowardly attempt to save his own life. Lin considers his argument, admits he's completely correct, then kills him anyway. His death is karmic in the sense that he had tried to make himself a prisoner of war so he wouldn't be shot, when he himself had murdered a prisoner of war against the wishes of Captain Brenner/O'Brian, causing the good guys to rebel against him in the first place.
This was the whole plot behind Guile's ending in Street Fighter II. He wants to kill Bison to avenge his friend Charlie's death, then his daughter Amy and his wife Jane dissuade him. Note that Jane was more concerned about Guile killing Bison in cold blood than asking him to come back home, until Amy chimes in and says that they both want him to return.
The Force Unleashed light side ending. Galen Marek (formerly Starkiller) rescues the leader of the newly founded Rebel Alliance, defeating both Darth Vader and Palpatine in the process. He's stopped from finishing Palpatine off by a Jedi who tells him that doing so will cause him to fall to The Dark Side. Despite the fact that said Jedi is a leader of the rebel alliance and who's very purpose is to KILL THE EMPEROR.
Knights of the Old Republic for Juhani and Carth's quests. Xor is a disgusting slaver who cheerfully took part in the Cathar genocide, killed Juhani's father, and tried to buy Juhani herself as a Sex Slave. Saul Karath was Carth's former commanding officer who betrayed the Republic and carpet-bombed Carth's homeworld. After defeating them, you can encourage them to "finish the job," or invoke the trope.
Sunry:You think I'm some kind of monster, don't you? All I did was kill a Sith! How many Sith have you killed? Dozens? Hundreds? Thousands?
Carth: You're talking about acts of war. We're just defending ourselves! You murdered your girlfriend in her sleep.
Subverted at the end of Dark Forces II: Jedi Knight after the final battle between Kyle Katarn and Jerec. Jerec taunts Kyle, "Strike me down and the power of the Dark Side will be yours! I'm sure you haven't forgotten, I was the one who murdered your father." Kyle replies that he hasn't forgotten, pulls Jerec's lightsaber into his hand, and then tosses it where Jerec can easily reach it. Jerec takes it up, screams, and makes a final desperate lunge, where Kyle easily cuts him down.
Discussed in Tales of Symphonia: Dawn of the New World, when Marta fatally cuts down a villain trying to kill Emil she starts to break down thinking she's just like they are. Emil reassures her with the distinction that the villain attacked out of rage while she did it to defend a loved one.
Commander Shepard of Mass Effect can address this subject on several occasions:
In Mass Effect 1, after helping resident Cowboy Cop Garrus Vakarian track down a Mad Doctor who'd previously escaped justice, Shepard can either encourage Garrus to just shoot him, or persuade him to arrest the doctor and bring him in to stand trial. If the player opts for the latter, he tries to attack, forcing them to gun him down. When Garrus points out the futility of trying to arrest him, when he just ended up dead anyway, Paragon Shepard uses the Gargoyles explanation, that while they can't always predict other people's actions, that doesn't mean they should stoop to the same level.
In Mass Effect 2 s/he can use this again, twice, during Garrus' loyalty mission, first by preventing him from killing a Dirty Cop. Played with in this instance, as Garrus still decks him. ("I didn't shoot him.") Later, Paragon Shepard can interpose him/herself between Garrus' rifle and the man who betrayed his team to their deaths. If Shep keeps this up, it turns out the target is already near his breaking point from remorse and will turn himself in afterwards.
If you don't bring the seaplane parts back to Christine and Jade in Imprisoned, your character has one of these moments after killing Kyle, and lets himself die in the lab explosion.
In Warcraft III, Uther the Lightbringer warns Arthas that "vengeance cannot be part of what we must do. If we allow our passions to turn to bloodlust, then we will become as vile as the Orcs". This is proved right as Arthas becomes obsessed with destroying Mal'ganis and upon defeating him with Frostmourne, becomes the commander of the Undead Scourge, who ironically replace the orcs as the antagonists of the series.
In Tides of War, Jaina temporarily becomes racist and almost wipes out ogrimmar. Kalec and thrall point out that if she does this she'll be no better than arthas (in fact, given that arthas was acting out of a misguided sense of compassion, she'd be worse.) Fortunately it gets through to her, and she calms down to the point where she's still mad, but no longer racist.
In Call of Juarez: Bound in Blood, William tries to prevent Ray from killing an unarmed bad guy with this and the story of Jesus forgiving one the murderers crucified alongside him. Ray's reaction? "The Lord forgave him... a cold-blooded murderer? Well, that's good to know." Then he shoots the baddie dead.
Played straight in Persona 3 when Ken tries to kill Shinjiro as revenge for Shinjiro accidentally killing Ken's mother. The response is along the lines of "If you kill me, you will be just like me", though he's more referring to the guilt Ken would feel from killing than the crime he would have committed. Of course, Shinjiro is still fully prepared to die. Not that it matters because Ken is righteous enough that he believes killers shouldn't live so he plans on killing himself after doing the deed, which Shinjiro immediately protests when he finds out. However, Strega interferes and it goes downhill from there.
Also played straight in Shin Megami Tensei: Devil Survivor with Keisuke when the party tells him that by letting Yama kill people considered "evil", he's just guilty as them.
Used rather cleverly in Diablo: killing its human host doesn't affect Diablo at all, so the hero tries to imprison the Lord of Terror in his own body. It doesn't work, and by the end of the second game he literally becomes Diablo.
Attempted by the preacher in GUN as he's pleading for his life, but subverted by Colton White in a Talk to the Fist moment.
Used in Oni: at the end of the penultimate mission, Konoko is given the choice of killing or sparing Griffin, with this trope hinted at.
In RosenkreuzStilette, after defeating Iris and reminding her afterwards that the reason she won was because everyone was cheering her on and the reason Iris lost was because couldn't believe in others nor love her fellow man, Tia tells her to give up. Since Tia is a good-natured girl, this possibly implies that Tia refuses to kill Iris because she doesn't want to be a murderer just like her.
In the Fallout: New Vegas DLC "Honest Hearts", siding with Joshua Graham and his quest to destroy the White Legs tribe (the villains) will end with him about to execute Salt-Upon-Wounds, the war chief of the White Legs and a man who's committed countless atrocities. You can then stop him with this argument. Interestingly, after you've talked Joshua down, you can kill Salt-Upon-Wounds yourself.
Going with Graham and then convincing him to spare Salt-Upon-Wounds' life is essentially the closest to the Golden Ending that you can get with the DLC (although being a Crapsack World there really aren't any true Golden Endings). The Sorrows lose their innocence, but at the same time they realize that they can be strong without being like the White Legs. At the same time, this ending is the only one in which Graham finds a measure of peace in his life for the first time since his days in the Legion. However, you can also request that Graham simply fight Salt-Upon-Wounds in a "fair" fight and at least let him die on his feet, which results in Graham's personal demons being at the least sated.
Final Fantasy XIII-2 has Noel Kreiss, who is constantly struggling with this when it comes to the villain Caius Ballad, who wants Noel to kill him. "Noel, born at the end of days. You understand the true meaning of life. But know this: you will kill me, Noel."
In the confrontation with Nobumasa in Yo-Jin-Bo, Nobumasa is quick to declare Ittosai as a killer Not So Different from himself. As a result, while all of the other guys end up killing Nobumasa in self-defense, Sayori urges Ittosai to refrain lest he prove Nobumasa right, and The Cavalry does the honors instead just in the nick of time to keep Nobumasa from skewering both of them.
Parodied in the Sluggy Freelance story arc "That Which Redeems." When demons invade the Dimension of Lame and start eating and killing people for fun, most people still feel that killing demons would make them no better than demons. Unfortunately, when Torg tries to organize a human resistance, he discovers most Dimension of Lame residents think stubbing demons' toes or throwing pies also makes them as bad as demons. Torg ignores their protests and cuts demons into little bits anyway.
Inverted in GastroPhobia, where the protagonist Phobia is nearly killed by a deer-turned-monster attempting to avenge his mother (who Phobia killed six years ago). But upon seeing that Phobia is herself a mother, the deer can't bring himself to kill her. Giving her an opening to stab him to death.
Played straight in Shadowgirls when Paul stops Lin from killing mad doctor, even if he has more reasons to see him dead as anyone else, because as a police officer he would have to arrest a killer.
In General Protection Fault, after Nega-Nick is captured, Nega-Trudy expresses a desire to kill him, but Nick tells her that it would make her no better than he is. Nick then goes on to say that his experience in the universe taught him about opposites (as he earlier admits, Nega-Nick is what he might have become), and killing Nega-Nick would easily put her on the same dark path as her prime counterpart.
Spoken by Elisa to Goliath in the premiere, as Goliath held the villain David Xanatos over the edge of his own building (although she compares the act to something the other main villain, Demona, would do).
The show elaborates in another episode, saying killing someone in the heat of battle was all right, but attacking a defenseless enemy with the direct intent to kill was wrong.
Goliath has to relearn this lesson alot, as there are a number of occasions after Xanatos where he almost kills someone in blind rage but is talked out of it. Although he can be excused for it, as his entire existence before coming to Manhattan was to fight and kill threats. You can't just decide something one time and rewrite what might be centuries of attacking on your first instinct.
Subverted in an episode of Mighty Max. Norman is facing down a rival barbarian who killed his family. The bad guy uses this line because it's the last card in his deck. Norman smiles and says "I can live with that", then knocks him off a cliff.
Used in the Dungeons & Dragons episode "The Dragon's Graveyard", where Hank refuses to finish Venger on the grounds that if he did, "We'd be no better than you are."
Subverted in Batman: The Animated Series. When Catwoman is about to drop the villain into a vat of acid, Batgirl calls out "If you drop him you'll be just like him." Catwoman replies, "Grow up" and lets go.
Appears once in the 2003 TMNT, when Angel stops Casey from killing Hun, but the rest of the series says it's okay to kill villains, and in fact the turtles do kill (or sort-of-kill) a number of villains during the series.
Used three times if not more in Beast Machines. One is when Optimus Primal was thinking of using the Plasma Energy Chamber to destroy Megatron and everything else technological on Cybertron, in order to stop his tyranny. Cheetor flat-out tells him "You fire this thing, and you're no better than he is!". Another time is after Botanica joins the Maximals and uses it as her justification to not get involved with the actual fighting: "The more we fight like Megatron, the more we become like Megatron". Fortunately she realizes that it's also her world and her fight, so she joins the battle in time to save the others from Obsidian and Stryka. A variant, though not actually about killing, when Tankorr is revealed to have the spark of Rhinox, and he is awoken to this, he decides both the Maximals and Megatron need to go and he, Tankorr, should be ruler. Cheetor decides to reformat Tankorr to make him Rhinox again, but Optimus stops him, saying "Rhinox has made... his choice. If we tamper with his mind, then we're no better than Megatron. Let him go."
In The Simpsons, the episode "The Curse of the Flying Hellfish" has Burns sending assassins after Abe, trying to drown his grandson, etc... and yet, when Abe has Burns cornered...
Flash: I'm trying to speak for Superman. [She releases him]
When Big Barda is ready to kill Granny Goodness, Martian Manhunter stops her. Interestingly, he only says that the Enemy Civil War needs to continue and does not mention this trope.
This is the case with Huntress in the episode "Double Date." She is about to kill Steven Mandragora, the mobster who had her parents murdered while she was a child right in front of her eyes. Then Mandragora's son runs out and the look in her face after seeing him clearly says that if she kills him right in front of his child, she'll be just like him.
In one episode of The Venture Bros., The Monarch actually invokes this on Dean for tattling so Dean won't spill the beans about him breaking into Dr. Venture's lab just to screw around. And it works.
In The Smurfs episode "For The Love Of Gargamel", Papa Smurf tells his little Smurfs that leaving Gargamel and Azrael in their self-petrified state would make themselves no better than their enemies. It ends up becoming the justification for many a Save the Villain moment in the cartoon show, despite how ungrateful Gargamel ends up being.
The same reason why many families of murder victims discourage vigilantism.
Suffice it to say that this trope appears in debates over the use of violent measures in combating terrorism, or in any wartime situation, and leave it at that.
Basically, any time anyone kills anyone when it's not the result of a fight for purely self-defensive reasons, someone is going to bring this up. Even in self defense, people will bring this trope up, and you know the drill.
This situation came up after World War II. Nazis performed unethical and horrifying medical experiments on thousands of people. So what do we do with the results? Should we throw out all that data? Or use it? The trope was raised, but ultimately the decision was made to use the data.