An extreme version of Wouldn't Hit a Girl that extends to actually killing people.
For some reason an otherwise very good fighter can't kill or harm certain people — usually due to a Freudian Excuse, a curse, or some other prohibition — and this is almost always a gateway for this weakness to be used against them. If this fact is introduced early on, it's usually a Chekhov's Gun and will almost certainly be used sometime in the story.
Generally, if the person affected by selective slaughter is a villain the Flaw Exploitation is treated as ingenious on the part of the heroes, but if the person is The Hero, it's considered low. Yeah, a bit of a Double Standard. Unless of course it is done specifically to introduce a little grey to the heroic side. This very often results in a Go Through Me scenario.
- After fully awakening her Super-Powered Evil Side, Priscilla in Claymore literally walks right by the young Clare after killing the other Claymores, and it's explained that whenever she attacks a place the young girls are spared, as if she doesn't even see them. The regular Claymores are also prohibited from killing humans, though as Teresa explains, this is voluntary on their part, and that despite the consequences they can and sometimes do break it.
- Enishi in Rurouni Kenshin is unable to harm young women and becomes violently ill if he tries due to what happened to his sister (and he was witness to) and this becomes a crucial plot point twice: He didn't really kill Kaoru, he just abducted her and left a very realistic dummy to serve as a body. In the final fight, Kaoru throws herself between Kenshin and Enishi's second-in-command who was going to kill him. Enishi sees his sister in her place, and rushes in front of her and knocks his second-in-command out.
- The Peacemakers from Scrapped Princess can kill everyone except for Pacifica, the person they have to kill, because she was genetically engineered to be as much like their original master as possible.
- A slight variation: Kurogane in Tsubasa -RESERVoir CHRoNiCLE- is put under a curse by Princess Tomoyo that will weaken him a little bit more whenever he kills someone. The idea is that it will teach him the value of life, as he lacked that as Tomoyo's personal ninja, but he sees it more as an inconvenience for most of the story. It's later revealed that it wasn't actually a curse at all, but a protective spell, and she just lied about that in order to keep him out of trouble.
- In RoboCop 2, the title character is prohibited from shooting children. The child criminal villain takes advantage of this restriction to shoot him.
- And previously, in the first movie, he was prohibited from harming employees of the company that made him.
- The Predator, in the second movie, attacks a subway train full of people and kills only those who were armed except for the policewoman, whom was spared because he could see that she was pregnant.
- Subverted in Demolition Man. Wesley Snipes' character, Simon Phoenix, is prevented from causing any harm to Raymond Cocteau due to subliminal programming instilled in him while he was in suspended animation, soooo ...
[Simon points gun at Cocteau's head but cannot pull the trigger]Simon: "Now that I have an army, I don't need you anymore! Never liked you anyway.. like an evil Mr. Rogers."Cocteau: "Yes, you do! The programming prevents your harming me."Simon: [hands gun to other guy] "Will you please kill him? He's pissing me off."
- In Django Unchained, Django is mad enough to kill every white person in and around Candyland (because every white person around Candyland deserves it), making a special detour just to off the white mastiff handlers who train large dogs to rip black people limb from limb, and did this to D'Artagnan, making this a case of stone cold vengeance. However, during the final showdown, he takes care to courteously ask the black slaves present to leave the house. Apart from the one who actually not only has benefitted from being a slave but was also just as much a tormentor of the slaves and the one who destroyed his plans and hopes of leaving with Brunhilde, Candy's Uncle Tom estate manager Stephen, who he saves for last.
- Professional "cleaner" Léon in The Professional has a strict "no women, no kids" rule when carrying out his jobs. This is part of the reason he helps Mathilda seek revenge against the drug dealers; they slaughtered her whole family, including her little brother.
- The Philosopher in Ian Banks' Transition, a torturer, refuses to hurt women or children. He notes that some would claim that it was because he once heard his father raping his mother, but he believes that it's because he's a gentleman.
- In Mercedes Lackey's Oathbound series, the sword Need has a geas that prohibits the wielder from harming women, and at least one villain has used a Gender Bender to get around this.
- Patrick Bateman from American Psycho seems to be unable to kill people who love him. He wants to kill his coworker, Luis Carruthers, but when he learns that Luis is in love with him, he's physically unable to do it.
- In Buffy the Vampire Slayer, Spike is implanted with a device that causes him agonizing pain if he attempts to inflict harm on humans. Demons, on the other hand...
- In a CSI: NY episode, a hitman (who's trying much too hard to present himself as a sophisticated assassin type) turns himself and the woman who hired him in after she found out new information about the death of her daughter and changed targets, because the hitman didn't want to kill a woman.
- Averted in Warhammer 40,000: The Chaos god Khorne used to be the god of martial honor and prowess, whose followers would not attack defenseless victims. Nowadays, "Khorne cares not from where the blood flows, so long as it flows", and the only distinction is that they'll leave defenseless targets alone only as long as there's a Worthy Opponent nearby.
- In Girl Genius, Von Pinn (basically a monstrous semi-robotic babysitter) chases after Agatha, but one of the students gets in her way. Said student is quite aware of the fact that Von Pinn is completely incapable of hurting her due to programming. When the Psycho for Hire steps in and kills the student for her, Von Pinn gets mad and tries to kill her.
- In The Order of the Stick, when a "Mark of Justice" was put upon him Belkar could not deal lethal damage to any living creature inside the boundaries of any city, town or village without setting off the mark's full punishment. This restricted him to killing things outside city boundaries, destroying undead, or knocking people around with Nonlethal damage. This gets used against him to trip it off.
- Used after a fashion with Oasis in Sluggy Freelance. Her brainwashing causes her to automatically attack anyone she recognizes as a Hereti Corp employee. Several Hereti Corp agents get around this simply by saying "I quit." She can still kill them, but unless they do something do piss her off she has no reason to. The only person she's truly incapable of killing is Torg.
- Jack Noir, one of the villains in Homestuck, is unable to attack Jade after Bec was prototyped, as Jack gained Bec's affection for Jade along with his power. This is used by Dave during a fight.
- Brock Sampson of The Venture Bros. cannot kill women or children. He's not above killing anything else under the sky with gusto, and he'll even maim women (considering he plucked out his lover's eye), but his mentor taught him not to kill women or children as it's basically the only moral high ground they can actually claim over their enemies. They're also probably just not covered by his license to kill (even though the undead apparently are).
- Traditionally the Japanese ghost known as the onryo will attack everyone around it except the man who wronged it in life. This doesn't mean the man gets away unharmed, however, as usually the onryo is targeting the people close to him, and eventually the survivors will figure out who the onryo is going after...