Backstab Backfire

No second chances. I'm that sort of a man.

—The Tenth Doctor, Doctor Who, "The Christmas Invasion", the lying bugger that he is.

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 rage and charges after the hero, attempting to stab 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.

Examples of Backstab Backfire include:

Anime and Manga

Dragon Ball, Frieza Saga. 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 to get blasted by Frieza again. One more blast from Goku ends the fight.
- Also happened in the Red Ribbon Army Saga, specifically the fight with Mercenary Tao. 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 kill him.
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 Mobile Suit 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?
Vash:Beat My finger is still on the trigger.

A Certain Magical Index: 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 stab Yomikawa; Accelerator promptly snaps and proceeds to rip Kakine's arm off and smash him into nothingness.
Hilariously subverted 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.

Comic Books

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.

Film

Prince Caspian in the film of the same name.
Kung Fu Hustle
Gaston in Beauty and the Beast (although original production materials did indicate that this may not have been a backfire)
Jenner in The Secret of NIMH, literally stabbed in the back by his minion.
Captain Hook in Hook. And he would've had Peter, too, if not for Tinkerbell and a not-quite-dead alligator...
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. The Sheriff tried to attack him from the back but Robin Hood pushed his sword back and by accident stabs the Sheriff.
Bad Boys
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.)
Scar tries to shift the blame to the hyenas before his final confrontation with Simba in The Lion King. Unfortunately for him, the hyenas overhear and they aren't very pleased, as well as hungry...
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.

Literature

Happens occasionally in Redwall, notably in the fight between Logalog and Hogspit in Long Patrol.
Subverted in The Silent Blade by R.A. Salvatore. 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, although even Drizzt acknowledges that his loss has more to do with bad luck than any lack of skill on his part. Entreri doesn't care, 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—his goal, as it turns out, is to alert Drizzt to 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.

Live Action TV

In Doctor Who, the Tenth Doctor did this to his first opponent using a satsuma. Which is a small fruit to the uninitiated.

"No second chances. I'm that sort of a man."

- Imagine what he could do with a pointed stick.
Mystery Science Theater 3000 episode Cave Dwellers has another variant: The Hero's sidekick, who's been off Behind the Black or some such during the Big Fight, shows up just in time to kill the Villain as he tries to backstab the Hero.
This happened at the end of a Law and 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. Also, Dean sold his soul to accomplish this. Season Two finale teaches Sam that mercy does not pay. Season Four finale teaches him that cruelty pays worse, although logically it mostly taught him, again, that he should never trust anybody except Dean, ever, especially bad guys. Guy cannot win for losing.
  - That is to say, Sam wound up enabling the first and second steps of the Apocalypse through an act of heroic morality. He then actively executed the third through what he thought was making sacrifices for the greater good. He then solved it all by Heroic Sacrifice, and spent the next year and a half wandering around without his soul, kicking puppies. In case one of them might be an interestingly possessed puppy. Or just for something to do.
  - And now he hallucinates the Devil all the time. And doesn't have to worry about whether he can trust anyone besides Dean, because everyone is dead. And also there are man-eating unkillable shapeshifting monsters taking over the world.

Video Games

In the Video Game Flushed Away.
Navarro, when he tries to kill Drake after their battle at the end of Uncharted: Drake's Fortune.
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. As Oswald turns away, Skuldi prepares to blast him in the back with magic, but just before he casts his spell Oswald turns back and cuts him down with one blow, furious that Skuldi was willing to use Gwendolyn as a tool.

Oswald: You must have mistaken me for a better man.

Dark Forces 2: Jedi Knight has this in the light side ending.
- 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 did a variation: He claims that he would make the bubble traps go away, but when Megaman.EXE spares him, Bubbleman instead makes the bubbles start to accelerate their countdown time.
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.
This is basically the entirety of Mercenaries 2. The first 10 minutes or so are you pulling a job in Venezuela. The guy that hires you then decides he'll pay you in bullets; you only catch one in the ass as you go diving out the window. The "backfire" basically consists of you leveling half the country in your quest to take him down.
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 City this is the last mistake The Joker makes. He's about to die of a fatal disease and Batman has the cure. Batman voices his doubts as to whether or not he should give the cure to Joker with how many horrible things he's done and will continue to do. Joker takes this as meaning that Batman won't give him the cure and attempts to stab him forcing Batman to drop the cure ruining the only chance for Joker's survival and then he dies. What makes this worse is that Batman would have given him the cure even with his doubts.

Batman: Do you know something funny? Even after everything you've done...I would have saved you.
The Joker: That is actually...pretty...funny.

Western Animation

Avatar: The Last Airbender: Commander Zhao attacks Zuko after losing a firebending duel. In the grand finale, Ozai attacks an indecisive Aang, but is effortlessly restrained. Aang still doesn't kill him, instead using a new skill to permanently de-power him.
- Notably, in the first example, it's not Zuko but Iroh who blocks the backstab—and then proceeds to effortlessly push the disgraced Zhao halfway across the field, showcasing that he's far more competent than his jolly exterior indicates.
Happens to Starscream all the time throughout all Transformers continuities.
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 Series Finale of Jackie Chan Adventures, Uncle and Tohru casted a chi spell to banish Drago into Demon Realm. As Drago held himself to avoid falling, 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 to win the Evil Versus Evil the two demons were in before Uncle and Tohru casting that spell. It backfired when Shendu grabbed Drago's tail, pulling him down.