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The Radix: Metzger may be a cold-blooded hitman, but he would never shot someone in the back.
In Gone, Caine is understandably appalled at what Penny did to Cigar in the twelve hours she had him. To put it simply, she tortured him to insanity. When Lana regenerates his eyes he thinks back to some of the hallucinations she caused, and we get to see them.
In the Hive Series about a school of evil villains, villians like Dr Nero disprove of needless violence when it comes to being evil.
Tom Walker in Washington Irving's story The Devil and Tom Walker is lacking in redeeming qualities, but when he first makes his Deal with the Devil and the Devil proposes that he serve him through the slave trade, Tom immediately refuses, saying he won't have anything to do with that. He then eagerly accepts the Devil's second proposal, which is that he become a ruthless Loan Shark who ruins the lives of those around him. It's been suggested that this might have been meant satirically toward the stereotypical Northern businessman, who like Tom, was a greedy, unscrupulous miser, but abhorred slavery.
The Mallorean has a deeply chilling example in the fourth book. Belgarion finds a prophecy written by Torak, the villain of the first series. It reveals exactly what Zandramas, the current Big Bad, is planning (in essence, creating a new god of darkness). At the end, Torak has added a personal message to Belgarion... which says, in part, "If you're reading this, you've already destroyed me. What is foretold in these pages is an abomination. Do not let it come to pass." As Belgarath notes, Torak was stunned out of his madness long enough to feel revulsion at what he foresaw.
A less serious example is the spies within Drasnia making it a point to not spy on Queen Porenn when she's breast feeding her baby. Naturally Porenn uses this time to talk with her chief spy in total privacy by dressing him as one of her maids.
Also in his Elenium novels, after being talked into helping the newly restored Queen officially, Platime (who was the leader of all the criminals in the country) agrees so long as he's given a full pardon, when asked for what crimes, he admits to committing every crime except barratry, treason, and carnal knowledge of an animal, and the barratry was only because he didn't know what it was.
Platime's counterpart in Emsat, Stragen, who plays a bigger and nobler role in the sequel The Tamuli, has standards of his own. An actual bastard, he knows what it's like to be at the bottom of the social pecking order, so he trains and orders his guild to target only the nobility.
Terry Pratchett likes doing this in Discworld, especially with his Assassins' Guild. Like Leon, they do not accept contracts to "inhume" women (though they let women be Assassins, as of Men at Arms at least) or children, nor do they ever work for free, and they cannot accept contracts on someone who cannot defend himself (though "rich enough to hire bodyguards" qualifies as "capable of defending himself").
In Witches Abroad it's stated that Genua's branch of Assassins all left years ago because "some things sicken even jackals".
In Hogfather the head of the Ankh-Morpork branch is horrified by the excesses of Psychopathic Manchild Jonathan Teatime and frightened by his unpredictable actions. Teatime later hires a bunch of criminals who are also scared and repelled by him; they did kill people, but unlike Teatime, only when it was necessary. The scene that introduces Teatime notes that Lord Downey, leader of the Assassins Guild, does not have actual morals, but he does have standards. Teatime... doesn't.
In Guards! Guards!, the dragon is rather disturbed by Lupin Wonse's plan to use human psychology so that the citizens of Ankh-Morpork will begin to grow used to the idea of having to sacrifice young maidens to the dragon, and bluntly replies to Wonse that dragons "never burned and tortured and ripped one another apart and called it morality."
Moist also abhors violence (not only the violence done to him, but doing any violence himself to try to prevent it being done to him, to the extent that he absolutely refuses to carry weapons of any kind) while Gilt has no qualms about having people brutally murdered right and left and employs a particularly terrifying killer to do it for him.
It's deconstructed with Mr. Pump's speech to Moist, however, in which he points out that Moist's standards didn't stop him from ruining people's lives, kicking those who were already down and hastening their deaths anyway, even if he's not aware of it — just because what you do doesn't directly kill people doesn't mean you're not harming them, and just because other people are doing worse and more evil things than you doesn't mean that what you're doing isn't still evil.
When Mr. Pump tells Moist he has killed 2.030317 (or some such number) of people, that causes Moist to reconsider the morality of his actions.
Also, with the Assassins' Guild: Sir Samuel Vimes asks one of their assassins, after having failed (for the nth time) to kill Vimes at his booby trapped home, why they just don't shoot him down in the street. The assassin is horrified, "Like a common criminal?"
Vimes finds out in a later book that he's been taken off the register. Aside from being slightly disappointed at that at wondering if he can appeal, he reflects that the Assassins only take someone off the books if killing them would cause too much political chaos (Vetinari being the only other person they will not accept a contract on). They seem to have found another use for him though, as a training exercise for over-confident students.
We learn (in Going Postal) that Igors have a tradition and rules about making a break for it when The Marthter starts going off the deep end. You make sure the larder is full and everything is all tidy before you go, and it is, apparently, permissible (but not encouraged) to suggest that other, particularly likable servants might like to take a holiday in a different town right now. (Igors know that there's no percentage in being around when the pitchforks and torches come out.)
There's also the Thieves' Guild, which takes a very hostile attitude towards unlicensed theft. The Guild also requires thieves to stamp a receipt for the "customer", so the same people aren't hit too often or for too much. They also strictly refrain from killing victims, although that's more to avoid trespassing on the Assassins' economic turf than for standards' sake.
In The Fifth Elephant, It's insinuated that even Serefine von Uberwald was horrified by some of the actions of her son Wolfgang, which included altering the family tradition of Hunting the Most Dangerous Game so that the human prey had no chance at survival and murdering his infant sister because she wasn't able to change form like "proper" werewolves.
Thud!! has Chrysophrase of the troll mob, who doesn't deal in drugs. Well, not any more. And not the bad drugs, the kind that kill their users or cause them to become psychotic. Commander Vimes isn't impressed. Also during that book, one of his goons makes the mistake of making an indirect threat toward Vimes's family ("He knows where I live." "Yeah, he does."). Later, Chrysophrase says the threat was not on his orders, and the offending goon has been...dealt with, and incidentally would Vimes like a rockery for his garden?
In Eric, Astfgl's attempts to run Hell like a corporation disgust even the other demons.
In The Last Hero, Evil Harry Dread has quite an extensive list of criticisms directed at modern-day villains who don't follow The Code. Yes, Harry may be a bungling Punch Clock Villain who's never rated better than a Shed of Doom, with a donkey Steed of Terror and a hamster for his Right-Hand Cat, but at least he plays his role properly.
In Jingo, 71-Hour Achmed's name comes from the fact that he killed someone 71 hours after sharing hospitality with him, one hour short of the customary three day truce following such an act. This makes him anathema and outcast in the normally thoroughly amoral D'reg society. But he doesn't care. He did it because his quarry was an infamous Complete Monster, and he figured the criminal would strike the moment time was up, so he broke tradition and got the jump on him.
It's stated in Good Omens that even demons find certain actions unthinkable, including using holy water on another demon.
Ironically, Noble Demon Crowley is the one who crosses that line, though it's done to save his hide more than anything else. Later on, Crowley rejects the idea of tormenting Hastur by playing the tape he's trapped on in his Bentley until he becomes a Queen song (It Makes Sense in Context) because even a demon can only sink so far.
Demons like Crowley also have this view about Satanists. Most of them (like the Sister Mary and the other nuns) are actually harmless—having been raised in the faith for generations they're like your average non-observant Christian, acting ordinary six days per week and then attending a token black sabbath without being particularly devout. Some Satanists, on the other hand, like the ones you sometimes hear about on the news...
In the Incarnations of Immortality series, Perry, when serving in the position of Satan, is this. While he is the absolute master of Exact Words deals, he always honors his agreements, which is much more than he can say for his recipients, who take and use the gifts then try everything they can to weasel out of the deal they made.
bully Henry Bowers and his fellow gang members just barely allow Patrick Hocksetter to hang with them, even though he fuckingterrifiesthem. Bowers eventually has enough and tells Hocksetter that he's quite well aware of the abandoned fridge that he kills animals with, which eventually leads to Hocksetter getting attacked and devoured by It.
Henry's cronies Victor Criss and "Belch" Huggins are fine with beating up smaller kids, but they are shocked when Henry tries to carve his name on Ben's stomach with a knife (though this maybe has more to do with them being afraid of getting in trouble than moral objections). Victor also has an internal monologue in which he's fine with putting fireworks in Mike's shoes, but using the powerful ones that could blow his feet off is going Too Far.
In Layer Cake, one of the gangster characters has a sex shop as a "legitimate front" and has a practice that if anyone asks for child pornography, he will arrange a covert meeting and then will beat them to within an inch of their life.
Sirius Black's evil, pureblood-maniac parents, who disowned Sirius (after he ran away) when he was sixteen, didn't join Voldemort because they thought he was going too far. Sirius's own brother, Regulus, did join Voldemort, but then when Voldemort tortures a loyal house-elf, turns against him.
Godelot Sr, writer of Magick Moste Evile, wouldn't touch the subject of Horcruxes. The notes in Tales of Beedle the Bard indicate that he was a Card-Carrying Villain.
Terry Pratchett does this again in Nation, where even cannibals revile First Mate Cox.
Sam from Villains by Necessity could be a poster child for this trope. An assassin, who is explicitly stated by the text to be an agent of Evil, Sam still refuses to kill anybody who isn't his target while on an assignment. He also refuses to steal from his targets, and hates rapists so much that they are the one exception to the above no killing rule. This is in a book where the villains are the good guys, so it all works out rather well.
In the Tales of the Bounty Hunters anthology, there's a Boba Fett story. Apparently Jabba gives Leia to the bounty hunter for the night, as a reward. Fett is disgusted by this, but doesn't send her back. He gives her the bed and stands near the door, and tells her that sex before marriage is immoral and Han Solo is evil for smuggling spice—spice, in Star Wars, being anything from a drug to a rare medicine to an unusual food additive. Leia calls him out on this—one, he's a bounty hunter, essentially assassinating people for the prices on their heads, and two, he's working for Jabba the Hutt, who does a lot worse. Fett says that morality does not enter into that, because what he does is legal. Leia doesn't press it.
In Allegiance, the heroic Mara Jade does not generally get on well with Darth Vader, who always suspects her of trying to replace him. Still, she's got a much rockier relationship with the Imperial Security Bureau, saying in the narration that she knows that they are a Necessary Evil, but there's all too much evil and not enough necessary. And they do go after a stormtrooper for refusing to kill unarmed civilians; plus, they try to kill her. At the end of the book, while they're trading warnings, Mara sees that Vader doesn't like the ISB either, though probably for very different reasons.
In the X-Wing Series, Kirtan Loor has a couple moments like this. He's petty, puffed up, and vindictive, but when Isard talks about how the Krytos plague will decimate the Sullustans to such an extent that it might be best if they set aside some breeding stock for when the plague has run its course, Loor is taken aback and feels uneasy. The narration says that while he does consider Sullustans to be inferior, talking about them like they're grain to be poisoned for rats, with some pristine kernels held back, is a bit much. He's also sickened in General Derricote's plague lab when he sees the disease working on test subjects, and orders what he assumes to be a stricken mother and child to be taken away and cured, although he does hastily tell the General that this is part of the plan to drain the New Republic's resources. Later he becomes the leader of a terrorism front that detonates speeders filled with explosives in health centers and public places, but when his new boss orders a school to be bombed, he's horrified. His new boss sardonically mocks him. Here he is, not wanting to kill children, and yet he's performing strikes to keep people away from health centers, meaning that the Krytos Plague, which doesn't spare the young, will be less impeded.
In Legacy of the Force, Tahiri Veila chooses to offer Luke's son Ben sex in exchange for information rather than torturing him. Either option is disturbing when you consider that he's just turned 14—but, knowing 14 year old boys, is surprisingly good psychology...
In Bring Me the Head of Prince Charming, the demon Azzie replies to a little girl who mentions using the model guillotine he brought her on puppies:
I am evil, but I am not cruel to animals. There's a special Hell reserved for those who are.
In The Tales of Alvin Maker books by Orson Scott Card there's a quick procession of these with the latest baddies. First is the rather vile riverboat captain, who would not stoop to killing innocent (white) children. He's killed by Mike Fink, who in turn finds that he can't bear to stand and watch while their mutual employer, William Harrison, massacres a village. Finally Harrison, the worst of the three, tells Calvin that while he might be a dirty scumbag, at least he never sold his own brother out.
Although it's implied Mike only left because the hex his mother left him protected him from the curse leveled on the perpetrators of the massacre.
In Mistborn: The Original Trilogy, there are only two people Zane won't kill- Vin and his father. The former is because Vin's the one person the voices in Zane's head don't continually goad him to kill, while the latter is for no other reason than, in Zane's words, "a man shouldn't kill his father".
In the Haft Awrang book "Chain of Gold," one of the stories is that of a man who, overcome by lust, mounts a camel. This appalls even Iblis, who curses him.
In the Dale Brown novel Plan of Attack, Russian Army General Nikolai Stepashin, who sees nothing wrong with sneak nuclear bombings on North America, is disgusted by president Gryzlov's nuking of a Russian airbase to take out American infiltrators and the man's refusal to check for Russian survivors. This most likely contributes to his Redemption Equals Death later.
The Destroyer series of novels tells of the House of Sinanju, who have been assassins to the governments of the world for five thousand years. The current Master, Chiun, has been an assassin since childhood (and is over a hundred) and has perhaps half a million or so kills to his credit all done with his bare hands. But he kills child-killers and people who train children as terrorists without pay, since it's immoral to involve children in "the Games of Death".
"Gentleman" Johnny Marcone is a self-described professional monster. He directly controls all of the organized crime in Chicago and most of the police are fine with this as Marcone's desire to keep his house in order has actually resulted in crime going down since Marcone took over the mobs. He is shown to have a hand in numerous illegal and immoral enterprises... yet he draws the line at anything which exploits children and personally executes anyone he finds out has been dealing drugs to minors or pimping out children in his city.
Queen Mab, Queen of Air and Darkness, is a cold and ruthless woman who if you injure her, she will kill you. If you betray her, she will make you beg for death. If you make a deal with her, she will keep up her end of the bargain. It may not be the nicest, cleanest means in fulfilling her end of the deal, but, like all Fae, she will honor it to the letter of the deal. It is for these reasons Harry considers her the least Evil of his options when his back is broken and his daughter will be killed soon. He could call on a Fallen Angel or some evil Necromacy ritual, but those would turn him into a truly evil monster. Mab would make him one in time, but he would only have to kill as she commanded it. And like many weapons, she wouldn't use him callously.
The more malevolent of the Fae, including the above mentioned Queen Mab, can be considered this as a whole as the Fae have a sense of morality based on deal making. Honor the deal and you do well. Break the deal or renege and you will suffer.
"When we employed the Heart of Ahriman to bring a dead man back to life," Orastes said abruptly, "we did not weigh the consequences of tampering in the black dust of the past. The fault is mine, and the sin. We thought only of our four ambitions, forgetting what ambitions this man might himself have. And we have loosed a demon upon the earth, a fiend inexplicable to common humanity. I have plumbed deep in evil, but there is a limit to which I, or any man of my race and age, can go.
The Druids of his own isle of Erin had strange dark rites of worship, but nothing like this. Dark trees shut in this grim scene, lit by a single torch. Through the branches moaned an eerie night-wind. Cormac was alone among men of a strange race and he had just seen the heart of a man ripped from his still pulsing body.
The Secret Histories series has Mr. Stab, a Jack the Ripper Expy who loves to carve up his victims. When he sees the torture the conspiracy group Manifest Destiny has inflicted on several magical beings they captured, even he is horrified.
Mr Stab: "There's only one monster here, and for once it isn't me."
Flashman is, and admits he is, evil in many ways...but even he has lines he will not willingly cross.
"A scoundrel I may be, but I ain't an assassin, and you will comb my memoirs in vain for a mention of Flashy as First Murderer." (Flashman's response to a clear hint that, if all else fails to turn him aside, Flashman is to bump off John Brown. Later, he says nearly the same thing about a proposal to have him do in Emperor Theodore of Abyssinia.)
The narrator in Edgar Allan Poe's The Tell-Tale Heart when he refuses to kill the old man until he is awake "for it was not the old man who vexed me but his Evil Eye." (though granted his standards are insane.)
In Scorpia Rising, while Zeljan is discussing his Evil Plan with the rest of Scorpia, he calls DamianCray a madman.
Earlier in the series, one of the senior members of Scorpia, Max Grendel, attempted to retire from the terrorist organisation, disturbed by the fact that they were developing a biological weapon that was designed to specifically target children.
In the novelization of Demolition ManSimon Phoenix is shocked that Cocteau had Associate Bob castrated to curb Bob's ambition. Phoenix is a murderous bastard, but taking a guy's balls just isn't right!
Even at his worst, Artemis Fowl II won't stand for mistreatment of the environment He also won't kill people. Lemurs, maybe, but not people. When Holly compares Artemis to the villain Jon Spiro, Artemis uses this fact as his defense.
In Gilded Latten Bones, Garrett ponders this trope when he asks Sarge and Puddle, two of assassin-for-hire Morley Dotes's criminal associates, about the doings of TunFaire's resurrection men, and both are repulsed by the notion of stealing dead bodies for necromantic research.
In Warrior Cats, there is one villain called Mapleshade who is so screwed up all the other villains are scared of her.
In The Pool of Fire Will is horrified by a German town in which criminals are given to the Tripods to be hunted for the crowd's amusement. He notes that in England executions are regarded as an unpleasant chore, not a sport.
In in the Gor series rape and slavery are fine. On the other hand raising a girl from infancy with no concept of the outside world, sexuality, violence, or even men and then raping her is still viewed as horrific. That it drives the girls insane and they have to be killed the next day is probably a contributing factor.
In the novel In Enemy Hands, Citizen Captain Vladovich is so vicious he is unpopular even with other State Sec members.
Likewise, in Honor Among Enemies, even the crew of the PNSVaubon is disgusted by theacts of Andrew Warnecke's pirates.
In the BattleTech novel 'Star Lord,' it's implied that there's rogue mercenary and pirate bands who turned down offers to work with an otherwise charismatic and surprisingly well heeled leader not unlike a Bandit King when they discovered he was a direct genetic descendant of the setting's greatest and most infamous monster looking to finish what his ancestor started. These pirates are generally taking slaves, burning villages, and otherwise being barbaric...and they still don't want to be associated with this guy. Some groups eventually did join his army, in a flat aversion of this trope.
In Death: An example of this occurred in Betrayal In Death. There was a trio of thieves named Naples, Gerade, and Hinrick. However, when Hinrick found out that Naples planned to have people working for Roarke murdered, he pulled out, greatly enraging Naples. Hinrick doesn't deal in murder, because he considers it rude.
Sven Hassel writes of soldiers in a penal battalion, some of the roughest, cruelest, most degraded men in the forces of Nazi Germany...but sometimes they run across things done by the Nazis, or the Soviets, that repulse even these hardened, callous killers. When they get a chance to express their disapproval of such things in concrete form, it can get...messy.
Vince in Dean Koontz's Watchers is a ruthless killer, but he's also a rather considerate guy who has a code of honor and a clear sense of empathy for others. However, he is Ax-Crazy and his code is based on Blue and Orange Morality, so it really doesn't make him any less evil or any less terrifying.
Three witches from Christopher Moore's Fool admit being evil incarnate but they say to preffer staying away from politics - apparently even crushing toddler's skull is better than it.
Simon Heap, when he has shoot down Nicko with a ThunderFlash:
Don't worry, I don't harm family.
In Darke, Linda decides to invoke You Said You Would Let Them Go on a pair of lovebirds after holding one hostage to make the other bring Jenna to her. The Witch Mother stops her because a witch must keep her Darke bargains, and Linda seems to be forgetting the Rules.
Residents of the Capitol have no problem with watching children as young as twelve murder each other for entertainment. But when Peeta "reveals" that Katniss is pregnant, they go ballistic. Also, while they are known to demand sexual favors from the winners (one of many downsides if you're a winner who's on the attractive side) doing so if he or she is underage crosses the line (at least they say that publicly). In the first book, it was explained that after Finnick won at fourteen, "they couldn't really touch him for the first year or two. But ever since he turned sixteen..."
Lord Tywin Lannister of Casterly Rock will not shy away from monstrous acts, but he does them according to an ice cold calculation of benefits and he prefers them carried with equally cold blooded efficiency. He reacts with disgust to the murder of the young Princess Rhaenys not because a young girl was murdered, but because his brutal Mook Lieutenant who did it stabbed her something like 50 times when, as Tywin puts it "Anyone with the wits the gods gave a turnip would have spoken some soothing words to her and then smothered her with a pillow."(For context, Princess Rhaenys was a girl who was maybe 5 years old and had been found hiding under her father's bed while the capital was being sacked.)
The Guild of the Faceless Men will only kill their targets, not bystanders or even bodyguards.
The Ironborn hate slavery, although they have a very flimsy definition of it. Indentured servitude and forced prostitution: okay for them, but buying and selling people? NEVER!
Victarion Greyjoy has no problem whatsoever with kidnapping a woman to be his 'salt wife' (forced prostitute). But when he finds out his brother Euron has impregnated her, and so he must kill her, he breaks down in tears and swears vengeance on Euron.
Ser Jaime Lannister is, at least in the early part of the series, a pretty nasty fellow, but he was still known for this trope enough that the Starks ruled him out of the attempted assassination of Jon Arryn because it was felt Ser Jaime would look down on the use of poison.
Ser Bronn of the Blackwaterwould hurt a child. He'd want a decent price for it, though. It's the right-thinking sellsword's code: no nickels and dimes for the big stuff — contract agreed upon beforehand!
In the Vorkosigan Saga, one of the main industries of Jackson's Hole is making clone-slaves; including clones that allow rich men to achieve immortality by transplanting their brains and throwing away the old one, thus killing it's personality. Nearly everyone outlaws that. On Jackson's Whole there isn't any law.
In the Belisarius Series, Narses practically lives this trope. He betrayed Empress Theodora who was practically his adopted daughter. But when told to assassinate the family of a Rajput chief, he refuses and almost becomes a hero.
In Gene Stratton Porter's Freckles, Black Jack has agreed that Wessner can kill Freckles, however he likes, once they are gone, but he objects to watches while Wessner torments him, especially since Freckles would beat him in a fair fight. Another man is angry that Wessner didn't just keep Freckles from seeing any of them.
"You see here, Dutchy," he bawled, "mebby you think you'll wash his face with that, but you won't. A contract's a contract. We agreed to take out these trees and leave him for you to dispose of whatever way you please, provided you shut him up eternally on this deal. But I'll not see a tied man tormented by a fellow that he can lick up the ground with, loose, and that's flat. It raises my gorge to think what he'll get when we're gone, but you needn't think you're free to begin before. Don't you lay a hand on him while I'm here! What do you say, boys?" "I say yes," growled one of McLean's latest deserters. "What's more, we're a pack of fools to risk the dirty work of silencing him. You had him face down and you on his back; why the hell didn't you cover his head and roll him into the bushes until we were gone? When I went into this, I didn't understand that he was to see all of us and that there was murder on the ticket. I'm not up to it. I don't mind lifting trees we came for, but I'm cursed if I want blood on my hands."
Parodied in the Ciaphas Cain novel Duty Calls. Cain considers the uniforms of the PDF so garish that even Slaaneshicultists would find them distasteful.
This trope appears in Israeli poet Natan Alterman’s Summer Celebration. Miriam Helen asks notorious robber Misha Barkhasid to help her fight off Woldarski, the man she eloped with who turned out to be a dangerous criminal. Woldarski has threatened to mutilate her face with acid if she doesn’t work as a prostitute for him; when Barkhasid hears this, he says that a man of honour can live on robbing trains and the like, but ‘will never live off the profits of a woman’s body’.
In the Star Trek: The Next Generation novel Imzadi, the Sindareen leader of a raid on Betazed has no qualms about theft, violence and killing if necessary but his captive Deanna Troi's empathic abilities tell her he won't rape her.
Troi : "You're not a rapist. A thief, yes. A killer as needed. But not a rapist."
In Valor's Trial by Tanya Huff, the Others hit the ground battle from space with some sort of strategic warhead that fuses the entire battlefield and everything on it into smooth volcanic glass. According to an Other lieutenant that Torin Kerr teams up with later on, at least part of the Others' own ruling council considered this weapon horrific and tried to prevent its use.
Rainbow Six: Dimitriy Popov is an ex-KGB agent contacted by the bad guys to foment terrorism in Europe, which he does with enthusiasm for a rather large paycheck. The bad guys, who are planning human genocide on a planetary scale with only a handful of select survivors, eventually decide to let him in on their plan and include him in the chosen elite. He's so horrified by their plan that he runs away and rats them out at the first chance he gets. His information is instrumental in stopping them, effectively saving humanity.
In the first book, we find Locke Lamora risking his life, his treasured vengeance—-and he's a vengeful sort from a vengeful people and his freedom to warn his enemies about something that he thinks is just plain wrong. And The Thiefmaker is perfectly willing to kill children—-but he always makes the correct offering to the gods for the murder, and absolutely won't sell a child to slavers for any amount of gold. The Thiefmaker's attitude may partly be Pragmatic Villainy; being The Fagin is tolerated, but dealing with slavers is not, and the authorities are generally very serious about that.
In the second book, Red Seas Under Red Skies, he goes out of his way to trash and rob Salon Corbeau, a city dedicated to letting the decadent rich do as they please, because he's disgusted by one of their pastimes.
Damon Runyon's characters are generally pretty blase about crime, but there are things they would never, ever do. In the story "Gentlemen, the King!" three New York City hardboys recruited to kill a king in Europe abort the mission instantly when they find out that the King in question is a child... and end up killing the man who sent them, instead.
In The Lord of the Rings, orcs are Always Chaotic Evil servants of the Dark Lord who routinely kill and eat civilians, and are constantly waging war against each other. "Kindness" is a dirty word to them, and they torture people for fun. But eating other orcs is one thing they won't tolerate.
(Jim) Rawlings was a burglar and a thief, but like much of the London underworld he would not have anyone "trash" his country. It is a fact that convicted traitors in prison, along with child molesters, have to be kept in seclusion because professional "faces", if left alone with such a man, are likely to rearrange his component parts.
Older Than Steam: In Dr Faustus the freaking demon Mephistopheles tries to convince Dr Faustus that selling your soul for magic power is a bad idea. Several times, actually. He explicitly points out that a few years of screwing around with physics isn't worth the loss of eternity with God.
In the novel The House of Silk (the new Sherlock Holmes novel by Anthony Horowitz) we learn that even the Napoleon of Crime himself (never named in the book but it's obviously him) has standards he finds the child sexual abuse practice in the eponymous house so appalling he gives Watson information which directly assists in solving the case. The section where this happens is very well-written and despite this moment of dog-petting the reader is left in no doubt that Moriarty is a very sinister man indeed.
In The Box of Delights, Abner's henchman Joe objects to keeping clergymen and choirboys prisoner at Christmas, partly because he hopes it would "tell in our favour, if ever we come to be tried", but also because it's "not Christmas dealing".
In the novel Bloodhype, the AAnn commander on Repler is a proud member of a xenophobic imperial race that would cheerfully eat humans for dinner were it not against the terms of their treaty, but even he is appalled when confronted with Dominic Rose, a drug lord (and distributor of the eponymous narcotic) who would gladly sell out his own species for survival.
In A Clockwork Orange, the gangs may be brutal rapists and robbers, but they have a strict prohibition on having more than five members in each gang; Billy Boy has six, which is the main reason Alex holds him in such low regard. Also, it’s implied they completely refuse to kill anyone; at any rate, Alex is horrified when he learns he accidentally killed the old woman he robbed.
The Triads may smuggle drugs and serve as assassins, but they're not about to let the Old Ones take over the world so easily.
Michael Marsh claims he doesn't like hurting children unnecessarily. This is fifteen minutes before he tries to cut Matt's heart out.
Lohan has this reaction when he sees a vivisected child being prepared for use as a 'drug mule'.
Empire Of The East: Lord Chup has manipulated, schemed, and killed in combat, but he draws the line at throwing his wife to a demon. Not because he loves her (although he does); she tried to have him killed, and he would be perfectly happy to murder her face-to-face. But to regain her trust and then sacrifice her soul, simply to gain promotion, is beyond the pale.
In the sequel to Those That Wake, Arielle Kliest is fine with torturing Mal and threatening him to get what the Old Man wants—but when she sees the totality of his plan, which involves assimilating every mind on earth, she's horrified and betrays him.
Assuming that Patrick Bateman from American Psychoactually kills anyone, he's perfectly fine with killing men, women, children, and animals... but he can't bring himself to kill people who genuinely love him. He has no ability to return that love, but he won't kill them. This inadvertently saves a guy's life when Lou, misinterpreting Patrick's threats as a come-on, reveals he's gay and has a crush on Patrick; Patrick is sickened by the revelation but gives up on his plans to kill him.
In Edgar Allen Poe's story, "Bon-Bon," Pierre Bon-Bon is a chef, and drunkard prone to philosophical banter. One night, the Devil shows up at his restaurant, and Pierre invites Mr Scratch for a philosophical conversation over wine, hoping to glean some esoteric insight he can publish and making himself famous. During the conversation, the Devil explains how he sees and eats souls, and then proceeds to namedrop some of the many, many famous souls he's eaten, and how they tasted. Meanwhile, Bon-Bon has become roaring drunk, being unable to speak without hiccuping incessantly. Even so, he repeatedly offers to give his own soul to the Devil as a stew or a souffle, but the Devil repeatedly refuses, citing that he will not taken advantage of Bon-Bon's "disgusting and ungentlemanly" drunken state.
In Daemon, Brian Gragg/Loki is a JerkassCracker whose Establishing Character Moment is organising the date rape of a teenage girl at a rave and letting one of his associates take the fall for a data theft he committed. Yet even he is sickened when the Major executes Roy Merritt in front of his eyes and spends the rest of the two books doing his best to find and destroy the other guy.
In Alex Grecian's The Devil's Workshop,Jack the Ripper, of all people. Jack believes that murdering and mutilating people is part of a divine "plan," and his crimes are a kind of favor to the victims. Killing children, though, is right out, as they aren't "ripe."
This is a major theme in one of O Henry's short stories, 'The Tempered Wind'. Two conmen, Parleyvoo Pickens (the narrator) and Buckingham Skinner, team up with a third conman and set up a business in New York selling fake bonds. However, a newspaper report exposes the business as fake, and the conmen get a nasty surprise when their customers show up at the office and are revealed to be poor factory workers, disabled war veterans, old women, and even children. One woman tells them about how she had invested all her life savings and needs the money back for her dying child, while the factory girls are losing money for missing work, and one women is in tears because she was saving for her wedding. Pickens and Skinner give all the money back. When the reporter who wrote the article interviews the conmen again, Skinner says:
Me and Pick ain't Wall Streeters like you know 'em. We never allowed to swindle sick old women and working girls and take nickels off of kids. In the lines of graft we've worked we took money from the people the Lord made to be buncoed—sports and rounders and smart Alecks and street crowds, that always have a few dollars to throw away, and farmers that wouldn't ever be happy if the grafters didn't come around and play with 'em when they sold their crops. We never cared to fish for the kind of suckers that bite here. No, sir. We got too much respect for the profession and for ourselves.
The Groupmind AI from Royce Day's For Your Safety series is perfectly willing to lock up humanity in a Gilded Cage for eternity, but refuses to do physical harm to a single person.
It is implied that harm towards children is frowned upon by the mainstream villain community. Both Lucyfar and the Bull react negatively when Claire is threatened by Jagged Bones.
The head of the villain community, Spider, is known for her strict adherence to the rules of conduct between superheroes and supervillains.
The Apparition is firmly against turning the Inscrutable Machine over to the Council Of Seven And A Half's thugs on the grounds that the former are her friends.
In Honor of the Clan, Matt Prewitt, hired for a hit on the family of a group that defected to the Bane Sidhe, has no problems with a contracted murder of the family, but when his immediate boss, a complete psychopath, knowingly leaves a baby to burn to death instead of taking the time to put the child out of her misery, Prewitt puts two rounds in the boss's head because of it, telling the now-corpse "even I wouldn't leave a baby to burn, you sick son of a bitch".