This appears frequently in the Alien franchise, often by victims of facehuggers, although in a deleted scene from Alien Ripley finds her crewmates (who have all been attacked and either killed or dragged off by the titular alien) cocooned to the walls of its lair and the line is uttered as a request for euthanasia rather than to prevent the alien from reproducing.
Amen: The Doctor is shown complaining over the fact that the Catholic Church is ignorant about euthanasia after Bishop von Galen helped get Aktion T4 halted, and justifies having gassed mental patients on this basis.
Wolfstan gives one to Griff when he reveals that he's got the plague. Characters also discuss the use of misericorde at the battlefield.
Osmund does this to his lover Averill when he finds her insane and suffering. It is later revealed by Langiva that Averill's appearance of insanity was merely a temporary effect caused by drugs, rather than the result of being unnaturally brought back to life... maybe. This pushes Osmund over the edge.
In Blade II, one of the Blood Pack is bitten by a Reaper, and is quickly mutating into one of them. The others demand he be put out of his misery, so he's shot twice in the chest. However, he's mutated far enough that the silver bullets won't do the trick. Then a guy cuts half his head off, which also fails. Blade finally shoots a hole in the ceiling so sunlight will do the job.
Coupled with Fridge Brilliance in Cleopatra. One of Cleopatra's ladies-in-waiting serves her a poisoned drink and begs for forgiveness. Cleopatra says "I forgive you" and orders her to drink the poison. It's a Mercy Kill because being poisoned is likely to be a much more quick and effective death compared to what the servant might have gotten as punishment for trying to assassinate the Queen.
In The Descent Sarah finds Beth with an open wound in her throat (caused by an ice-pick) and ends her pain by smashing her head with a rock rather than leaving her to the Crawlers. She is understandably reluctant to kill her friend, but tearfully relents after Beth begs her to do it.
Invoked in Edge of Tomorrow, where the heroine insists that the hero should kill himself (or let himself be killed by somebody else) if he is K.O., crippled, or otherwise incapacitated. This is because doing otherwise might result in him being given a blood transfusion or just bleeding out, which would rob him of the time-warping power.
Fatherland: Pili is taught that people with disabilities should be killed on this basis. Xavier gently seeks to dispute this with a story of a disabled man who's revealed to really be an angel. This is the first sign that he's not on board with the Nazis entirely, and would be horrified at the Holocaust.
Tragically used at the end of David Cronenberg's The Fly (1986). The only thing Brundlefly can do is crawl miserably along the floor and point the end of the heroine's gun at its own head.
The hero of The Fly II had no choice but to mercy kill a poor dog. Why? The condition of the dog was a result of an experiment that turned the dog inside out, thus deforming it both physically and psychologically.
In Full Metal Jacket when the sniper girl has been shot and is begging the soldiers to shoot her again. Joker is the one to finish her off, and the Marines around him congratulate his cold-blooded killing, when the look on his face shows it was not a cold-blooded kill at all.
Parodied in Funny People, when Adam Sandler's character tells Seth Rogen's, who is working for him at the time, that he has an almost certainly fatal disease and asks him to shoot him, for a fee. When Rogen replies that he needs time to think about it, Sandler replies that he was just kidding and that Rogen is sick for even considering it. He DOES still have the deadly disease, though, so that part wasn't a joke.
Chevy Chase is offered a similar deal in Fletch, although the man who requests it doesn't really have bone cancer and is trying to use Chevy as an Unwitting Pawn.
In the 2002 Korean Film "H", a police officer executes her kneeling partner on the beach in front of their hire up. He had solved a serial killer case and the knowledge he gained tormented him.
Neil in Heat mercy kills one of his partners in crime after the guy has been tortured and is dying.
The Hunger Games: In the film, Cato actually screams "Please!" to be mercy killed by Katniss. And his ordeal didn't last 20 hours, unlike the book.
Nonhuman case in I Am Legend: After Neville's dog Sam is infected with the vampire virus, Neville chokes her to death. Borderline in that it's also self preservation, because Sam is already becoming hostile.
The French film I've Loved You So Long is about a woman who was in prison for murdering her son. In the end it is revealed that he had terminal cancer, and he got so sick and in pain that she killed him out of mercy, after spending one last day with him doing everything he loved.
In Land of the Dead, the protagonist shoots a woman who's being bitten in the neck by a zombie right between the eyes to spare her either being eaten alive or reanimating as a zombie.
Lust, Caution: When we first see Mr. Yee, he orders an underling to now finish off a prisoner who he has just finished interrogating, on the grounds that the Japanese didn't specify they wanted him alive. He says "give him a quick one" implying that by killing the man now they are saving him from more torture at the hands of the Japanese, who would eventually kill him themselves.
Monsieur Verdoux: It is hinted that the titular anti-hero, a fired banker who marries and murders wealthy widows in order to support his invalid real wife and their toddler son, may have mercy-killed his loved ones after losing everything in a stock market crash.
The events in Morgan are set up by Morgan coming across a badly injured deer while out in the woods with one of the scientists and breaking its neck.
Purgatory: Blackjack's brother is wounded in the initial shootout. Blackjack wants to just leave him to die slowly once he can't stay ahorse, claiming he can't spare the bullets to put him out of his misery. Sonny does it for him.
An unusual modern film usage features at the climax of Quantum of Solace when Bond prepares to shoot Camille, as they are trapped in a burning building and she is reliving her childhood trauma of being trapped in a burning house. Subverted when he finds a way of escaping their situation.
Double Subversion in the first Resident Evil movie. Rain gets infected by the zombie virus and tries to get the rest of the group to kill her before she turns, but they refuse and try to find a cure. Later on she becomes a zombie anyway and has to be killed.
Averted in The Return of the Living Dead, when two survivors are cornered in an attic. Knowing these zombies can't be killed by bullets, the male survivor covertly aims his pistol at the back of his teenage companion's head, rather than let her be eaten alive by her own undead boyfriend. Averted because the place gets nuked before he can pull the trigger.
In Saving Private Ryan, they give The Medic an overdose of his own morphine because he cannot survive his wounds. Inverted earlier in the movie, when an unnamed soldier orders the others not to mercy kill Germans who have been doused in flames.
Soldier: Don't shoot! Let them burn! note This could also be seen as ammo conservation or wanting to keep advancing forward.
In Saw III, Amanda does this to Adam as shown in a flashback. Adam was left in the bathroom to die at the end of the first movie, and Amanda, unable to detach herself emotionally like Jigsaw does, suffocates Adam with a plastic bag so he dies quickly instead of starvation or disease. It may have been more guilt on Amanda's part. In the flashback where she and Jigsaw are setting up the bathroom trap, it shows she failed to secure the key that would have freed Adam properly (Jigsaw told her to tie it around his ankle — she just placed it in the tub and it wound up going down the drain). If she had done what Jigsaw told her, Adam would have survived.
In a French 1974 film Les Seines de Glace (Icy Breasts), adapted from Someone Is Bleeding by Richard Matheson, Marc shoots Peggy, an incurable psychotic man-murderer he is in love with, to spare her from the asylum. This is decidedly different from the original novel, where they just elope together... only for her to perform Off with His Head! on him.
Mal shoots a man he had pushed off his hover-jeep who then gets dragged away by Reavers. Later, Zoe acknowledges it: "That was a piece of mercy."
In the same film (minutes after the Mercy Kill described above), Jayne gets skewered in the leg by the Reavers, and is hanging off the back of the mule, prompting him to make Mal promise to shoot him if the Reavers take him. Mal quickly takes aim, prompting Jayne to shout, "Don't shoot me first!" before Mal shoots through the rope tying him to the Reaver ship instead.
Also attempted (as seen on tape) by a woman on herself as the Reavers are breaking in. She fails. The tape continues to record.
In Sister Cities, the sisters' ailing mother Mary, who's suffering from ALS, asks her second-oldest daughter Austin to do this by crushing up pills and putting them in her applesauce and drowning her in the bathtub.
In Sky Captain and the World of Tomorrow, the protagonists come across Shangri-La in the Himalayas, but find out that it has been taken over by Totenkopf, and its inhabitants used as slaves and test subjects in the irradiated mines. They find this out from the last survivor of Totenkopf's experiments, a disfigured old man who asks but one thing for all his answers: "Kill Me." It is unknown if they fulfilled his wish.
Nathan Fillion is no stranger to the Mercy Kill. In Slither, he doesn't hesitate in delivering a headshot to a friend whom the mutated Rooker-beast has infected.
In Solace, this is actually the modus operandus of the killer. His victims are all terminally ill people to whom he grants a painless death. This is subverted by John Clancy who points out that most terminally ill victims would prefer to live just a little bit longer. However it turns out he's a hypocrite for saying so because he performed euthanasia himself on his terminally ill daughter, who was in great pain.
Soylent Green: A lot of people choose euthanasia ("going home") which is completely legal and freely available in special clinics. With the state of the world, it isn't difficult to see why.
In the film, Lieutenant Rasczak shoots one of his men who is badly wounded and captured. He then tells his troops that he expects them to do the same for him if it is ever necessary. It is.
In the book when a man who went AWOL during basic training murdered a little girl the rest of the recruits had to go and make sure he was hanged, because he was their man right or wrong. Rico begins to think whether or not they should try to cure him of his insanity, in his mind one would have to be crazy to kill a child for no reason, but then decided that living with the knowledge of what he did would be worse than death. So he kinda views hanging the guy as a mercy killing, or at least mercy for every other little girl he might have come in contact with.
When the Borg start assimilating crew members, it's Picard who takes it upon himself to vaporize, Tommy gun, or otherwise euthanize every affected crew member he can, because he knows what it's like.
Picard: You may encounter Enterprise crew members who have already been assimilated. Don't hesitate to fire; believe me, you'll be doing them a favor.
Of course, when he got turned, they certainly had to save him. Picard believed that the only reason it was possible for him to be rescued at all was because the Borg Collective deliberately left him some degree of autonomy, to act as an interlocutor, while he saw the redshirts as being unsavable.
Picard: There was no way to save him! Lily: You didn't even try!
In Star Trek V: The Final Frontier, Bones relives one of his most painful moments, where his father is dying and suffering from an incurable disease. He begs Bones to stop treatment so that he can finally die. Bones does so. What makes the memory especially terrible is that a cure was found three months later.
He was not feeling merciful. He was feeling calm, and clear, and he knew that to climb down that black beach might cost him more time than he had. Another Sith Lord approached....In the end, he was still Obi-Wan Kenobi, and he was still a Jedi, and he would not murder a helpless man. He would leave it to the will of the Force.
Stonehearst Asylum: It turns out this was Lamb's motivation in the "incident" which landed him at the asylum.
In Street Fighter, Guile's first reaction on seeing his friend Charlie mutated into a beast is to try a mercy kill. Dhalsim is able to talk him out of it.
Discussed in Tora! Tora! Tora! when Admiral Kimmel, watching the attack on Pearl Harbor, gets hit with a spent and harmless shell casing: "It would've been merciful had it killed me."
Happens in the horror film Train. Alex and Willy find their friends Sheldon and Todd locked in a torture car, both horribly mutilated. Sheldon can still walk, but Todd is barely alive, missing his eyes and unable to move. He begs his girlfriend Alex to finish him. When she tearfully refuses, Sheldon does it, cutting him with an axe.
Subversion in Transcendence. Martin, the first victim saved and hybridized by the nanomachines, is captured by RIFT so they can gain access to Will's code and engineer a virus to shut him down. In the process, they have to shoot him several times and isolate him in a Faraday cage so Will can't network with him. His wounds are severe enough that he dies on the table, since without the network connection the nanites can't heal him. They later rationalize this to Evelyn as giving him back his humanity, even though the whole time he was begging them to let him go so he could survive.
The Western Ulzana's Raid has a brutal example. Hostile Apaches menace a white woman and her child on a stagecoach. A cavalry officer arrives and, fearing the woman's about to be raped, shoots her in the head. Then, when the Indians turn on him, shoots himself for good measure.
Violet & Daisy: The Guy thinks of his own murder as this, given that he's dying of cancer. It's also discussed by Violet and Daisy, with them coming to his view. Daisy shoots him in the end.
In WarCraft, Llane orders Garona to kill him so that he might avoid getting his soul sucked out by Gul'dan, and so that she may gain respect among the orcs, which would give her a chance to forge peace between them and humans.
In The Whisperer in Darkness, Professor Wilmarth encounters the disembodied brain of Henry Akeley, who asks him to do this. In a subversion, Wilmarth cannot bring himself to carry out the request.
Done multiple times in The Wild Geese — the mercenaries don't have the time or resources to carry their incapacitated comrades, lest the Simbas arrive and overrun the whole company; given the Simbas' notorious reputation for brutality and butchery, a shot to the head is kinder than being captured.
You Don't Know Jack follows the career of assisted suicide advocate Jack Kevorkian, otherwise called "Dr. Death", who helped dozens of people with incurable diseases painlessly kill themselves, and eventually euthanized a disabled man at his request, leading him to be incarcerated (after winning numerous acquittals before).
Z for Zachariah: John shot a teenager who begged him to, and he thinks this may have been Ann's brother.