Selective Slaughter

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. This very often results in a Go Through Me scenario.

See also Never Hurt an Innocent, Wouldn't Hurt a Child, Kick Them While They Are Down.

Anime and Manga

 * 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:.
 * 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.
 * 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..
 * The Peacemakers from Scrapped Princess can kill everyone except for Pacifica, the person they have to kill.

Film
"[Simon points gun at Cocteau's head but cannot pull the trigger]
 * 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
 * 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: "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:"

Literature

 * 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.

Live Action Television

 * 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...

Theater

 * The titular pirates in the Gilbert and Sullivan operetta The Pirates of Penzance won't attack orphans (as they are all orphans themselves), which leads to Flaw Exploitation from everyone they capture.

Web Comics

 * 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 Order of the Stick,
 * 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.

Western Animation

 * Brock Sampson 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).
 * Clay from Xiaolin Showdown mentioned on two occasions that he would not fight (specifically, hit) girls.

Real Life

 * 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...