633 Squadron: Norwegian resistance leader Lieutenant Erik Bergman has been captured and taken to the local Gestapo HQ for torture and interrogation, so RAF Wing Commander Roy Grant goes over and delivers a precision strike with a single Mosquito, both to silence Bergman and to put an end to the torture.
In The Bull of the West, Determined Homesteader Ben Justin is forced to shoot his entire herd, including his prize Angus Aberdeen bull, after one of the cows contracts aftosa.
Cube Zero: Meyerhold is infected with a rapidly-progressing, flesh-eating virus. Haskell says they won't abandon him if he tests every room, but throws him into a trapped one at the first opportunity. Rains is angry with him for this, but Haskell is right that he was already a dead man and it was only a matter of time before he would have infected one of them as well if they stayed anywhere near him.
In Dark Touch, Niamh practices a twisted version of this, killing the children of the town so that they won't face the abuse she assumes they will suffer.
Daniel Craig's character in Defiance shoots his horse so that the people in his camp can eat again.
In Dragonheart, Einon can only be killed if Draco is slain because of their shared heart. Since Draco's half is the one with the lifeforce, Draco is the one who must be struck down. He orders Bowen to kill him, invoking the favor that Bowen offered to him long ago. Bowen initially refuses. When he sees Einon get up again as hateful as ever, he realizes that there will never be peace as long as Einon lives. Ultimately, Bowen has to kill his friend in order to save his kingdom.
In 2010's The Expendables, Barney is forced to kill Gunner when he goes rogue in a drug-induced homicidal rampage. Averted when it is revealed in the end that it was merely a Disney Death.
The Field Guide to Evil: In "A Nocturnal Breath", Xaver is forced to shoot the farm's only cow after she is bitten by the drude, and then burn her body. Later, he holds his beloved dog Wachter in his lap and slits the dog's throat after it has been bitten.
The Fly II: When Martin (Brundlefly's son) had to put a mutated golden retriever in horrible pain out of its misery by suffocating it with chloroform, after his employer promised him he put it down two years ago.
And of course, the original movie ends with Seth fully transforming into a giant fly and eventually becoming fused with the metal door, silently begging his girlfriend to give him a Mercy Kill, which she does.
In Fresh (1994), the title character shoots the dog, when it's clear he won't be able to look after it, after his friend who was taking care of it dies.
Full Metal Jacket, at the very end, when Joker delivers the Coup de Grâce to a downed Vietcong sniper who had shot two men down from said platoon and proceeded to torture them with more shots to their limbs to entice more of the platoon to enter the killzone to save them. She was also a teenage girl. The rest of the platoon wanted to let her slowly bleed to death, but Joker gave her a Mercy Kill.
Godzilla (2014): Joe Brody is forced to lock his own wife in a radiation-filled chamber to die in order to stop the leakage contaminating an entire city once he's informed that she's already been fatally contaminated.
In A Gunfight, Abe is forced to shoot his beloved horse in the head after it succumbs to rattlesnake venom.
The Guns of Navarone. Captain Mallory has discovered that Anna is a traitor and is forced by the circumstances to execute her. As he prepares to do so, Anna's friend Maria shoots her instead so Mallory doesn't have to.
Vampire movies tend to run into this a lot due to the inevitable moment when a loved one has to get a stake through the heart for everyone's benefit, but nobody has ever gotten as much mileage out of it as Hammer Studios. The setup typically involves the resident vampire hunter explaining to everyone else that the victim's soul is in agony as long as they are left in this state, and as brutal as it seems, staking them is much more humane than trying to ignore the facts and leave them in their coffins. Although it must be done, the staking is so unpleasant that it often serves to give the heroes a personal reason for seeking revenge against the vampire who "sired" their friend, getting the final act of the story in motion.
This was essentially the philosophical heart of the Jet Li wuxia movie Hero (2002) (Ying Xiong for Mandarin speakers). While most of the protagonists oppose the King of Qin, who intends to conquer and unify all the neighboring kingdoms, one of them realizes that the peace and prosperity of unification will far outweigh the short-term suffering of the war. It's a weird loop, in that Character A is urging Character B to let Character C Shoot the Dog, but there it is.
In I Am Legend, Neville hauls his dog (and only living/sane companion) Sam, who had been mauled fending off the infected, to his secret base, injects her with the only experimental cure that had even begun to show results, and hugs her. Then, as the hair loss and aggression become undeniable, he snaps the dog's neck. Cue Heroic BSoD.
The same thing happens in the Vincent Price version.
The John Woo movie The Killer (1989) is all about this trope, almost painfully so. The biggest example by far is Ah Jong having to put a bullet to Fung Sei, his best friend, so that he doesn't die like a dog after getting tortured and then shot by Wong Hoi and his people.
In the Kingdom of Heaven's Director's Cut, Princess Sybilla tearfullypoisons her own son to death, having found out that the child is afflicted with leprosy - therefore he's condemned to a life of incurable pain, like his uncle King Baldwin.
Inverted in Kingsman: The Secret Service. Prospective agents raise a puppy throughout their training. When few are left, a different agent than their advocate hands them a gun and tells them to "shoot the dog". Those who fail to do so are dropped from training due to not following orders, even seemingly cruel or unnecessary ones. Those who pull the trigger find out that the gun is loaded with blanks.
In Lean on Me, many of Joe Clark's actions have some arguable moral ambiguity to them, from chaining and locking school doors (in violation of fire safety rules) on being told that someone from inside the school let an expelled student into the building, to firing a teacher for picking up trash during the school song for which everyone was told not to move.
Joe: I cried "my God, why has thou forsaken me?" and the Lord said "Joe, you're no damn good. No, I mean this! More than you realize, you're no earthly good at all unless you take this opportunity and do whatever you have to." And he didn't say "Joe, be polite".
In The Mountie, Grayling has to shoot the brave horse who saved him from a lynching when it breaks its leg during the escape.
In Never Grow Old, Patrick has to put down his family's only horse after it gets sick. The sheriff offers to do it for him, but Patrick opts to do it himself.
The Trope Namer incident in Old Yeller was a Mercy Kill for the eponymous beloved dog who had turned rabid (and a coming-of-age moment for Travis).
In Patton, a pair of stubborn donkeys pulling a cart are blocking a bridge that Patton's troops are trying to cross, leaving the column strung out a long the road and vulnerable to the German aircraft that are attacking them. Enraged, Patton shoots both donkeys and has their bodies and the cart they were pulling thrown off the bridge.
At the end of Road to Perdition, Tom Hanks shoots Jude Law so that his son will be able to go through life having never killed anyone. In the original graphic novel, his son is the one who pulls the trigger.
In the previous scene, Tom Hanks guns down his father-figure and mentor Rooney (played by Paul Newman) because it is the only way to reach Rooney's Ax-Crazy son and avenge his dead wife and son. Hanks' character is visibly torn apart by killing Rooney, but has no choice at this point.
Done in The Sand Pebbles by Jake Holman (Steve McQueen (actor)). After Po-han is captured and is being tortured, the San Pablo is leaving the dock where Po-han is being tortured. Holman is ordered not to fire at the Chinese who are torturing Po-han, so Holman shoots Po-han to put him out of his misery.
Mal's choice to disguise Serenity as a Reaver vessel is also treated as a Shoot the Dog - he is desecrating the corpses of their deceased friends and turning their home into an abomination in order to get to the planet Miranda and blow the lid off the Alliance's darkest secret.
One of the alternate endings for Se7en had Somerset shooting John Doe for killing Mills' wife so that Mills won't have to go to jail for it.
The Silence (2019): In a fairly literal example, Hugh lets the family dog out of the car because his barking attracted the extremely sound-sensitive vesps to his family and will again as soon as they finish off Glenn, even though it means the dog is going to be their next meal.
In Star Trek: First Contact, Picard chooses to shoot a redshirt dead rather than let him be transformed into a Borg (a combination of mercy kill and preventing the creation of another enemy). He finds himself at the receiving end of some major What the Hell, Hero? criticism as result.
Anakin saves Palpatine/Darth Sidious from death at the hands of Mace Windu, but got Mace killed. He did it because Palpatine claimed he knew a way to save Padme from dying, but Palpatine really didn't know how, and Anakin thus crossed the line to The Dark Side for nothing.
In Rogue One, Cassian shot one of his informants because he had a broken arm and was thus unable to climb to escape the stormtroopers chasing after them.
Street Fighter: In a scene that feels wildly tonally dissonant considering the cheesiness of the rest of the movie, Guile tries to Mercy Kill his old friend, Carlos Blanka, when he discovers that Bison has turned him into a hulking, green-skinned monster. Dhalsim steps in and stops him, arguing that Blanka isn't evil, he just has trouble telling the difference, and Guile holsters his gun. Then the movie immediately goes back to being kind of a goofy action flick with Raúl JuliáChewing the Scenery whenever he gets the chance, and neither Blanka nor Dhalsim are seen or even mentioned again.
Sunshine (2007). After a fight with a fellow crewmember and an Important Haircut, Mace becomes determined to focus entirely on completing the mission (which, to be fair, involves saving the entire human race). He first wants to ignore the Distress Call from Icarus Iwhich turns out to be the far better thing to have done, and later volunteers to execute Trey so as to preserve what's left of their oxygen supply. Trey, as it turns out, is already dead.
In True Grit, Rooster shoots the horse when it is unable to carry on due to exhaustion, and he carries Mattie on foot.
This is how most of Vera's family views the situation once her work as a secret abortionist comes to light in Vera Drake. Reg, in particular, likens the abortions to mercy killings, since most of them would have been born to mothers who couldn't care for them.
In Vertical Limit, while trapped in an ice cave on K2, one character murders an injured comrade with a syringe of air, rather than share the drug they need to survive at such a high elevation.
In The Warlords, Jet Li is a general whose army has been laying siege to a well fortified town for over a year with neither side making any progress. The Empress tires of the stalemate and has given him a deadline of a few more days. His supplies are running out, and so is the food supply of the besieged town. Both armies are about to starve to death. Li's second in command negotiates a truce with the town's leader, and they agree to surrender the town to Li's army on the condition that they share their food with his people. Li however did not authorize this agreement and afterwards arrives at the cold reality that there is only enough food for his own army to survive for a few more days; were he to share that food with the enemy's troops, neither army would survive. He reluctantly orders the massacre of the entire town that had just surrendered to him. Unarmed, they are all shot down in a hail of arrows. Li's supplies were enough to sustain his army long enough to reach the next town.
In Winter In Wartime, Michiel's horse breaks a leg while he and Jack are escaping from Nazis. Jack Mercy Kills the horse
In X-Men: The Last Stand, when Jean Grey has completely lost control of her alternate personality "The Phoenix", Wolverine is the only one with the fortitude (both moral and physical) to put her down in the end, despite being in love with her. To take the edge off the trope, she regains enough control to request that he kill her. In the original comic saga, Wolverine pointedly can't bring himself to do it.
Subverted in Zombieland. Wichita asks Tallahassee and Columbus to perform a mercy killing on her "infected" sister, Little Rock, when she stops them, and insists that she be the one to do the deed. She then promptly turns the gun on the two men so that she and her sister can steal their weapons and vehicle.
