The Hero and The Big Bad have had their climactic battle. The Hero wins but chooses to spare the Big Bad, be it to prove he is better or because he doesn't want to kill (mostly also because the Big Bad was begging for his life). As the hero turns and walks away, triumphant, the villain lets out a scream of rageand charges after the hero, attempting tostab him in the back. The Hero decides that one chance to change was enough; he won't give the villain a second. He either kills the Big Bad or disarms the villain entirely.
Often sub-trope of Karmic Death; here it is not Karma but the Hero who did the killing. The hero often uses an Offhand Backhand in this situation. Thematically related to Never Bring a Knife to a Fist Fight: By showing his lack of moral integrity, the villain dooms himself to defeat. It also allows the hero to kill the villain without the hero killing the villain in cold blood, because that would be unheroic. Unless the hero is a Combat Pragmatist who doesn't have a problem with taking the most sensible but morally questionable course of action.
Compare Self-Disposing Villain. It should not be confused with Taking You with Me, which is where a character after being spared attempts to kill his enemy with full knowledge and/or intentions of his/her own death as a result.
In the fight between Perfect Frieza and Super Saiyan Goku, Goku spares Frieza twice. The first time, Frieza's been beaten so badly that he's not even a challenge anymore, so Goku tries to leave, but nearly gets sliced open. The second time, Frieza is cut in half by his own energy disc. Goku gives Frieza some of his energy to help him live, and starts flying away... only for Frieza to try to blast him again. One more blast from Goku ends the fight.
The fight with Mercenary Tao in the Red Ribbon Army Saga. Tao promises that he'll change his ways, but then throws a grenade at Goku as soon as the latter let his guard down. Goku then threw it right back at him, resulting in it exploding in his face.
Eis Shenron was a subversion. After being beaten by Goku, he begs for mercy. Nuova Shenron allows him to live even though he knows he deserves death under the condition that he surrender his dragonball. However, he decides to kill Goku in a sneak attack involving Ice blades. It's subverted because he actually DOES succeed in his backstab, blinding Goku from his ice blades. However, it still ultimately backfired due to Goku's other senses still being high enough for him to punch Eis through the gut and finish him with a Dragon Fist.
A weaker variety happens twice in Bleach fairly early in the Soul Society arc, to less-than-honorable Soul Reapers. One tried a textbook grovel-and-backstab; the other's credentials as a bully were repeated surprise attacks against an unthreatening (if valid) target. Both survive, as it's used for early establishment of heroic character rather than drama. The second, however, is robbed of his abilities permanently.
In Gundam 00, Lyle is in a position to take revenge on Ali Al-Saachez for killing his brother Neil but ultimately decides to give him one last chance. As he lowers his gun, Ali yells "DUMBASS!" and grabs his gun... then Lyle proves why he's called Lockon Stratos as he lands a headshot with a lightning-fast quickdraw, following up with a few more torso shots to make sure he's dead.
A somewhat convoluted example happens between Nena and Ribbons. After Ali kills her brothers, Nena is forced to work for Wang and by extension for Ribbons. The final straw happens when Ribbons turns out to have employed Ali, royally pissing Nena off. Nena promptly turns against them and only Hong Long's Heroic Sacrifice saves Wang's life from Nena's initial retaliation... or not: Nena catches up to the fleeing Wang and finishes the job with her Gundam. Then Ribbons reveals that he expected this kind of betrayal and retaliates with who Nena expects to be Ali but turns out to be Louise - the girl whose family Nena killed for absolutely no reason in the first season and who now brought a Super Prototype to bear against Nena's Gundam Throne. Backstab Backfire with the backstabbee never involving himself directly - exactly the kind one would expect from Ribbons.
In an early episode of Rave Master, Haru Glory spares the life of an adversary, citing Thou Shalt Not Kill. The bad guy tries to attack him from behind, only for Haru to hold his sword out and the villain to (non-fatally) impale himself.
Defied in Trigun: After pushing Vash's Berserk Button by targeting innocents, Monev the Gale gets brutally beat down. Vash, after a furious inner dilemma, decides to spare him and walks away.
Monev: Aren't you afraid I'll try to shoot you while your back is turned?
After Accelerator beats Kakine he is about to shoot Kakine who is lying on the ground when Yomikawa stops him. Kakine then uses his dark matter to stabYomikawa; Accelerator promptly snaps and proceeds to rip Kakine's arm off and smash him into nothingness.
Hilariously used in One Piece. After Luffy beats Foxy in their Davy Back Fight boxing match, Foxy offers Luffy a sporting handshake. Just as Luffy takes his hand, Foxy whirls about to perform his 'Overhead throw of vengeance'...Luffy's arm just stretches out, and Foxy ends up hitting his head.
After the Green Goblin killed Gwen Stacy, Spider-Man tracked him down and beat him nearly to death. Spidey was so angry that he wanted to kill the Goblin, but at the last minute stopped himself. He thought that Osborn was no longer a threat and Osborn, who was still able to remotely control his goblin glider, positioned it behind Spider-Man and hit the gas hoping to impale him. Spidey dodged the glider and it hit Osborn instead, killing him. At least, that's how the story originally went.
In the final issue of the Kitty Pryde & Wolverine limited series, Logan had defeated Ogun and brought him to his knees. He spared Ogun's life and began to walk away alongside Kitty. Ogun produced a knife and rushed at the two of them from behind. Wolverine noticed it in time to tell Kitty to use her phasing power, causing the knife to pass harmlessly through her. He then stabbed Ogun with his claws, killing the villain.
The Disney version of Peter Pan has Peter, after his climatic duel with Hook on the ship, in which he puts Honor Before Reason, take Hook's sword and threaten him with it, to which Hook says something to the tune of "Please, I Will Do Anything!" Peter complies, but only if Hook says that he is a codfish. Hook is only too happy to oblige and Peter decides to let him go. But when Peter crows in triumph, Hook raises his hook at Peter to backstab him with it. However, Peter, with his ability to fly, takes to the air in the nick of time, leaving Hook to lose his footing on the mainbrace where they dueled and fall right into the jaws of the waiting crocodile.
Additionally, during the two's faceoff Simba, Simba actually does have Scar at his mercy but instead chooses to exile him, repeating the last words Scar spoke to him as a cub. Scar instead sneak attacks Simba with some hot coal to try and put him off balance. Though it does wing Simba, he's still as combat adept as ever and eventually tosses him off a cliff down to where the above-mentioned hyenas are.
Captain Hook in Hook. And he would've had Peter Pan, too, if not for Tinkerbell and a not-quite-dead crocodile...
Hellboy II: The Golden Army. Twins Prince Nuada and Princess Nuala are linked; Hellboy defeats Nuada in a duel but spares his life (and thus, Nuala). When Hellboy turns away, Nuada starts to attack, only to be stopped by his sister, who kills herself (and thus, Nuada).
Technically subverted, as Nuada was actually telling Hellboy to finish him off when Hellboy defeated him, so he technically wasn't backstabbing.
Inverted in Ip Man, where the Hot Bloodedly-heroic Zealot Lin tries to attack the not-quite-evil General Miura while the latter has his back turned and gets fatally wounded for his trouble.
Played comedically Robin Hood: Men in Tights where Robin Hood spares the Sheriff of Rottingham after beating him and turns his back on him. When the Sheriff goes for the backstab, Robin is sheathing his sword and accidental impalement occurs.
At least one film adaptation of Cyrano de Bergerac changes the dueling match in the first act so that the title character wins without killing his opponent. Said opponent instead dies while attempting to stab him in the back after the fight's officially over. (Note that this does not apply to the original.)
Peter Stegman does this to Mr. Norris in Class Of 1984. As he is hanging from the rafters of the school, he pleads for Mr. Norris to help him, saying that he's "just a kid". When Mr. Norris extends his hand to help Stegman, Stegman tries to stab it with his switchblade in a final act of malice. It ends up causing him to fall and become hung by electrical wires over the school gymnasium.
In Alien vs. Predator, after "Scar" allows Weyland to live after discovering he has cancer, Weyland creates a makeshift-flamethrower and set Scar ablaze...only for it to be completely useless and makes Scar stab him in the chest.
Overlaps with Secret Test of Character in Taken 2; Bryan (after killing the current wave of thugs looking for revenge for the first film,) confronts the Non-Action Big Bad, Murad, and confirms that he has other sons who will perpetuate the Cycle of Revenge. While holding the Murad at gunpoint, he offers him a chance to call off the blood feud and just go home. Murad accepts, Bryan puts the gun down and turns to leave. Murad immediately picks up the gun and tries to shoot Bryan In the Back, only to discover that it's empty. Murad has time for a brief Oh Crap moment before Bryan kills him. Note that unlike a straight heroic example of this trope, the killing is not done in simple self-defence; the backstab was comprehensively defeated and the villain rendered completely harmless before being killed in cold blood.
Happens occasionally in Redwall, notably in the fight between Logalog and Hogspit in Long Patrol.
Inverted in The Silent Blade. Drizzt Do'Urden and Artemis Entreri have engaged in a duel to the death to determine once and for all who is the better fighter. Entreri ultimately loses (more because of bad luck than any difference of skill between the two fighters), and tells Drizzt to finish him since he cannot live with the knowledge that he was beaten. Drizzt refuses, and begins to walk away. Entreri runs at him from behind and cries out in rage; with the implied goal of alerting Drizzt of his attack so that Drizzt will be forced to kill him. And Drizzt does defend himself by turning around and stabbing Entreri. However, a protective spell cast on Entreri without his knowledge protects him and mortally wounds Drizzt instead... so, while Entreri was not playing this trope straight by trying to kill Drizzt, his attempt to get himself killed backfired. And he was furious.
At the end of Brotherhood of the Rose CIA-trained assassin Saul is sneaking up on the hotel where his surrogate father, Diabolical Mastermind Elliot, is staying...only to find Elliot standing out in the open waiting for him, clearly exposed to his fire. Elliot asks for an end to this conflict between him and his 'son', and to be allowed to retire peacefully. Saul nearly shoots him anyway, but realises he could have been killed while they were talking. So he agrees to a truce and follows Elliot into his hotel room where he gets jumped by Elliot's bodyguards. After all, as Elliot says, what's to stop Saul from changing his mind later on? This way he can dispose of Saul without alarming the hotel guests. Saul is able to overcome the attack and promptly submachine-guns Elliot to death. No point in making the same mistake twice.
Happens with alarming regularity in Wheel of Time. Mostly by the bad guys.
A notable one happens in one of the later books, when Galina (a Black Ajah) teams up with Faile (the wife of one of the three main heroes). Galina needs the Aiel Oath Rod, and promises Faile freedom if she gets the rod for her, all the while intending to betray Faile once she has her hands on it. After Faile does so, Galina collapses a building on top of Faile, and leaves her for dead. Just a few chapters later, Galina is caught by the one person that her Restraining Bolt compels her to obey, leaving Galina to live the rest of her life as a slave, with no chance of escape. The cruel (or funny) twist of fate is that if Galina had played things straight, she would have been rescued right alongside Faile, with the Oath Rod, and successfully pulled off a Karma Houdini.
Played with in the Honor Harrington series concerning the duel with Pavel Young. Young shot her in the back by turning and firing before they'd reached the proper places, and the referee's shooting of him in punishment (which ended up being coincident with Harrington's own return shot, while lying on the ground wounded) meant his attempted backstab would have resulted in his death even if he'd killed or incapacitated her.
Live Action TV
In Doctor Who, the Tenth Doctor did this to his first opponent without looking and using a satsuma. Which is a small fruit to the uninitiated.
This happened at the end of a Law & Order: Special Victims Unit episode entitled "Criminal". One man has a gun to the head of another, intending to kill him, but is eventually convinced to spare his life and let the police take him into custody for a crime he committed. As he walks away, the guy he wanted to kill picks up the discarded gun and attempts to shoot him in the back—only to be gunned down by a police sniper.
A villanous example occurs in the season 2 finale of the 2009 re-imaging of V. The Fifth Column convinces Lisa to kill her mother Anna (the supreme commander of all Vs), so that Anna's deposed mother Diana can re-take her role as Queen. To this end, they fake a kidnapping but leave clues for Anna to follow. When Anna arrives to the warehouse where Lisa is being held, she frees her daughter and turns around. Lisa quietly grabs a gun from a nearby drawer and points it at her mother. Anna sees it in a reflection and starts acting emotional, not letting in that she knows. She convinces Lisa that she has changed, and Lisa does not shoot. This is all a lie, of course. She kills Diana, imprisons Lisa, and forces her to watch as Lisa's sister (who looks like her) sleeps with Lisa's boyfriend and, possibly, kills him.
On True Blood, Sam Merlotte gets into a fight with Marcus Bozeman, with both wanting to kill the other. Sam wins and chokes Marcus almost to death, but holds back at the last minute, opting instead to hit him with a Reason You Suck Speech and telling him to live with those truths before turning his back and walking away. Marcus is pisssed and goes for a gun lying nearby. He's tackled by Alcide before he can shoot Sam. Alcide accidentally hits him in the throat and kills him.
Sort of Pyrrhic delayed-action one in Supernatural; Jake successfully backstabs Sam to death after being spared. Sam's brother brings him back to life. Sam shoots Jake repeatedly in the face next time he sees him. By this time, Jake has already opened the literal door to Hell - and Dean had to sell his soul to resurrect Sam. While neither Jake nor his Evil Mentor live to see it, their plan still succeeds and several seasons of destruction follow.
You can do this yourself in Arcanum, when you receive the Mastery in backstabbing.
Subverted in Odin Sphere after the boss fight between Oswald and Skuldi the Wise Man. Skuldi tells Oswald where he sent Oswald's wife Gwendolyn to as part of his plan to lure Odin out, and begs for his life. For a moment Oswald turns away, and Skuldi prepares to blast him with magic - but Oswald, furious that Skuldi was willing to use Gwendolyn as a tool, turns back and cuts him down with one blow, and makes it clear he never had any intention of letting Skuldi live.
Oswald: You must have mistaken me for a better man.
Notably, the main character, Kyle Katarn, is Genre Savvy enough to know that the villain would attempt this, and for that reason, gives the defeated villain his weapon back. Predictably, the villain attempts to strike him down with it, whereupon Kyle cuts him down without mercy.
Bubbleman.EXE performs a variation with a I Surrender, Suckers before hand: he claims that he'll make the bubble traps go away after being beaten and claims he is sorry and never will do it again, but when Megaman.EXE spares him, Bubbleman instead accelerates the bubbles' countdown time. but thankfully Protoman.EXE appears and finishes Bubbleman off before he can can do any harm and he and his operator give Lan and Megaman a lecture on never lowering their guard against enemies even one's that are groveling at their feet.
In Star Wars Battlefront: Elite Squadron, X2 had defeated X1 on Mustafar, X1 declares that X2 should kill him and thus turn to the dark side. However, X2 decides not to, with the DS version that he'd rather "lock his pieces up". This wasn't the answer X1 was looking for, however, so he levitates his discarded lightsaber, ignites it, and then attempts to kill X2. Unfortunately for him, X2 saw it coming, backflipped over the blade, resulting in X1 accidentally impaling himself, and then proceeds to fall off the edge into a flowing lava river.
A variation occurs in Star Wars: The Force Unleashed. After Palpatine was defeated by Galen Marek, Palpatine urges Marek to submit to the Dark Side and then kill him using the Dark Side. Marek, with some help from Kota, ultimately refuses. However, Palpatine doesn't try to backstab Marek. Instead, he tries to attack Kota for interfering, resulting in Marek holding back the Emperor's attacks long enough for the rebels to escape safely. It counts as a backfire because although Palpatine ultimately survived, those actions also ultimately robbed him of a proper replacement apprentice of Darth Vader, since his holding back the Emperor's powers (as well as bearhugging the Emperor and then proceeding to detonate most of the throne room) killed Marek in the process.
A villanous example in a Jade Empire flashback, where Emperor Sun Hai and his brothers Sun Li and Sun Kin raid the Dirge in order to take the power of the Water Dragon. While Sun Hai is draining her power, his brothers try to kill him while he's distracted. Unfortunately (for them), he's already absorbed so much power that even being impaled doesn't stop him. He proceeds to kill then reanimate Sun Kin, turning him into an unstoppable enforcer, while Sun Li escapes, leaving his family to be slaughtered by the Emperor's assassins.
Also, Li's betrayal of the Player Character. While he does kill the protagonist, the latter rediscovers his legacy as a Spirit Monk, comes back to life, and uses his new powers to kill Li.
In Batman: Arkham Citythis is the last mistake The Joker makes. Batman has the cure for the Joker's blood poisoning, but asks why on Earth he should save the life of someone who's caused so much death and destruction. Joker backstabs Batman (apparently intending to steal the cure), but this causes Batman to drop the vial, robbing Joker of his last chance. As he dies, Batman makes one final observation:
Batman: Do you know something funny? Even after everything you've done...I would have saved you.
Also happens in the DLC Harley Quinn's Revenge. When Batman and Gordon are distraught by Robin's apparent death in the Steel Mill's explosion, Harley - who was just neutralized and dragged outside by Batman - pulls a knife on him. It shows just how the Joker's death affected her judgment.
In a flashback in the Batman Beyond episode "Ace in the Hole", Bruce Wayne, while making his pilgrimage to the site where his parents were murdered, was ambushed by a Jokerz thug who wanted to mug him. He effortlessly beat him, and then told the Jokerz to leave. The Jokerz thug wasn't finished yet, so he tried to attack him again while Wayne was walking away. However, Wayne wasn't the one who stopped him: Ace stopped him.
In the pilot of Gargoyles, Hakon tries to stop Goliath from attacking him by blaming the Captain for the Wyvern Massacre. The Captain, enraged by Hakon's attempt to frame him, wrestles him off the cliff.
In the Grand Finale of Jackie Chan Adventures, Uncle and Tohru casted a chi spell to banish Drago into Demon Realm. As Drago clung to the edge, he pleaded for help from his father (Shendu), who agreed to help him while reminding Drago Earth was Shendu's world. Drago then took advantage of this to pull Shendu into the abyss and win their Evil Versus Evil. It backfired when Shendu grabbed Drago's tail, pulling him down.