In Axis Powers Hetalia, Kiku Honda/Japan does this to his adoptive older brother Wang Yao/China. We don't see it because of a Gory Discretion Shot, but it can be assumed that poor Yao tried to run away when he saw Kiku pointing the sword at him. Later on we can see the scar.
On The Galaxy Railways, Manabu's brother dies when a space pirate shoots him in the back. Later, Bruce is killed when a thug he pissed off earler in the episode shoots him in the back while he's buying a drink from a soda machine.
Juubei attempts to kill Kazuki this way in Get Backers, but Kazuki dodges very slightly at the last second.
Mazinger Z: In the Gosaku Ota manga continuity, Dragon with an Agenda Archduke Gorgon murdered Big Bad Dr. Hell by stabbing him from behind with one sword when he was distracted. Afterwards he pulled his sword from the body, said "So long, old madman" and left him lying -and bleeding- on the ground, laughing all the way as he left the place.
Fullmetal Alchemist has a literal example, when Lust stabs Havoc in the back after having pretended to be his sweet and loving girlfriend.
The penultimate episode of the Revolutionary Girl Utena anime contains an extremely painful and jarring scene where Anthy stabs Utena with her own sword—the same one that Utena wielded throughout the series. And it's made even more tortuous for the audience by Utena's drawn-out struggle to remain on her feet and save Anthy despite her betrayal.
A very brutal version of this happens to Beelzemon in Digimon Tamers; in one of their fights with the D-Reaper, he gets an opportunity to rescue its hostage, Jeri, and goes for it. He's so desperate and focused on saving her to make amends that he completely forgets to keep his guard up. The D-Reaper notices and shoots half a dozen razor-sharp flying discs into his spine. OUCH. He survives, but barely, and his injuries are so severe he's sidelined for the rest of the war.
In Chapter 483, Sasuke tries to kill Sakura this way. It doesn't work, thanks to Kakashi.
In Chapter 678, Black Zetsu backstabs Madara right in the middle of his Evil Gloating. Zetsu then reveals Madara was just one more cog in a machine run by Princess Kaguya.
There's a scene in End of Evangelion where a JSSDF commando sneaks up on a NERV gate guard, grabs him by the face and pushes a knife into his back, quite horrifying as you see the guard arch his spine, groan and then go limp, and in another scene we see one gun down NERV staff whilst they're running away from the JSSDF with their backs turned
From Bleach, it's actually recommended procedure for Shinigami to attack Hollows from behind. This a mixture of pragmatism and a desire to avoid feeling guilt on seeing the Hollow's human face.
A slightly earlier and weird example is what happens with Starrk and Wonderweiss. Turning up behind the man, Wonderweiss puts his hand through Ukitake's chest, causing Kyoraku to go for him by flash-stepping behind the 'kid'. But before Wonderweiss gets hit, Starrk takes the opportunity of the distraction to shoot Kyoraku in the back. Kyoraku, being a fine Combat Pragmatist himself, later returns the favour.
When the sixth squad come to arrest Rukia in season one, Ichigo tried to intervene and fights the sixth squad lieutenant Renji Abarai. Just as Ichigo is winning, captain Kuchiki Byakuya Flash Steps in from behind and breaks Ichigo's sword. As Ichigo turns to face him, Byakuya Flash Steps again, now behind Ichigo and stabbing him in the back right through his heart, ending the fight and leaving Ichigo to die! On a show where most fights last several episodes, this one is over in seconds. In fact, it all happened so fast that Ichigo was on the ground before he even knew he'd been attacked.
Krillin has attempted to do this with his Destructo Disk against Vegeta in his Oozaru form (it failed) and Frieza (after his first transformation), where he only managed to lob off some of the latter's tail. Though if the villains hadn't reacted RIGHT when they did, Krillin could've easily killed them both, despite them being vastly stronger than him.
Frieza himself tries this. Goku beats him and now he's not not half the despot he used to be. So he tries to blast the Super Saiyan from behind. Not smart, Frieza. Just not smart at all.
In Gundam SEEDNice GuyNicol Amalfi dies while trying to stab Kira YamatoIn the Back; Kira turns around and proceeds to cut Nicol (not The Blitz but Nicol himself) nearly in half. Nicol's action is never portrayed as anything other than sympathetic, given that a) it's a war, b) The Blitz was too damaged to confront Kira and The Strike head on, and c) he was trying to save his best friend, Athrun, from Kira.
in A Certain Magical Index, one teleport esper, Kill-Point, can only teleport directly behind people. As such, this is his preferred attack style.
Gino Weinberg of Code Geass certainly isn't above this: this is how he kills Senba of the Four Holy Swords, and how he destroys the otherwise highly guarded Shinkirou much later.
While not done in the back, the titular character of Jonah Hex is shot and killed during a cardgame when he stops to clean his glasses.
Supporting character J.D Hart was shot in the back when he turned away, refusing to draw on a woman he had no quarrel with.
A controversial moment among fans in Judge Dredd was the death of the original Judge Giant. Attempting to arrest an agent of East Meg One, he is distracted by said agent's Robot Buddy. The agent rather calmly shoots Giant in the back.
Happens to Zorro of all people. How? In The Lone Ranger: The Death of Zorro #1, a sixty-something Diego is compelled to don the cape and mask one last time to help Indians brutalized by American soldiers. Only thing: "Tonight you forgot to watch your back, old bastard!", while fighting skillfully a sergeant
In one issue of the Secret Six comics, Deadshot and Catman have a argument about this that ends with Catman insisting that Deadshot walk in front of him on all future missions. Given Deadshot's particular skillset, personal history and belief that a surprise headshot is the kindest way to kill someone, it's not a terribly unreasonable demand.
In Earth 2, Steppenwolf stabs Wonder Woman in the back with his sword.
Many fights in Christian Humber Reloaded involve one protagonist trying to get behind the other. In Vash's first fight with his corrupted self, he manages to get behind his corrupted self, only for his corrupted self to then get behind him.
This trope is dealt with in the Hunger Games fanfic Some Semblance of Meaning. When Obsidian is leaving the pack, he ends up stabbing Ford, the guy from District Four, in the back. This comes back to haunt him, and later on, in his fight with Achilles, the latter mocks him for being a backstabber. Then, when Obsidian has him prostrate on the ground and is preparing to kill him, Achilles asks him to just let him turn over first, so he won't stab him in the back, and lets him stab him in the chest instead.
At the climax of The Secret of NIMH, Sullivan pays Jenner back for stabbing him by throwing a knife into his back, killing him.
Disney Peter Pan. Hook is sneaking up on Peter with sword drawn, when Smee sees him and calls out, "Capt'n!" Peter then turn around and asks, "In the back, Captain?"
In Disney's Tangled, Mother Gothel does this to Flynn/Eugene when she fatally stabs him in the back with her dagger, unseen, while he's trying to save Rapunzel (though, Gothel may or may not be a Dirty Coward, yet she does fight dirty). Guess he should have seen that one coming.
Near the end of Beauty and the Beast, the Beast lets Gaston live after he wins their fight. He goes over to Belle, who has just returned. At that point Gaston takes the sneaky road and climbs up to the Beast so he can stab him from behind. When the Beast lashes out after getting knifed, Gaston subsequently loses his footing and plummets to his death.
Sheriff Wade Addams: It's the worst kind of killer that would shoot a man in the back just as soon as look at him.
Dirty Dingus Magee
Charles Stuart: Here he comes! Shall I shoot him in the back when he passes? Herkimer 'Hoke' Birdsill: No! I want this to be a fair fight. I'll shoot him in the back. It's the Code of the West.
Subverted in Ip Man, where Zealot Lin tries a sneak attack on General Miura after the latter had accepted the Chinese pugilists' loss and gets a fatal beating for his troubles.
Very strangely applied in Dial M for Murder. Margot stops the hitman trying to strangle her by grabbing a pair of scissors and, instead of sticking them in one of the dozen or so really good places to stab somebody who's right in front of her, decides to laboriously reach all the way around and plunge them into his back instead. It's probably safe to blame this one on the Hays Code.
In the John Woo movie The Killer, when Inspector Li confronts the title character at the beach house, he demands that he turn around, as he won't shoot people in the back. The title character, a Hitman with a Heart, notes that this is another way that they're the same.
In L.A. ConfidentialCaptainDudley Smith tells Ed Exley he's not ready for detective work because he's not willing to shoot a suspect in the back. By the end of the movie, Exley has done just that... to Smith.
Hickey uses this in Last Man Standing, putting his tommy gun down and turning his back to his victim preparatory to drawing a pistol and blowing them away. And then in the final showdown:
Hickey: "I don't suppose you're the kind of man who would shoot an unarmed man in the back?"
The Operative pulls this on Mal in Serenitywhen the latter is trying to get to Mr. Universe's backup broadcast equipment, causing Mal to ask if he's made him angry (referring to their conversation in the Companion training house).
As soon as Order 66 is brought up, the troops accompanying Aayla Secura (that blue Jedi girl) shoot her in the back. Over and over. To be fair to the clones, she was standing in front of them, and they probably had been with her for a while so they know that firing at her from the front is a dodgy proposition
All of the clones shot their respective Jedi in the back. Mundi turned around at the last second, but the thought is what counts. Which was the entire point of not just Order 66, but the entire clone wars. To weaken the Jedi (and Jedi senses) far enough that when you shoot, their back is still towards you when the bolt hits.
Subverted with Yoda, who was not having any of that backstabbing. Yoda was aware of what was going on as the Jedi were being slaughtered, far too many to simply be attributed to combat, and when the clones approached him with weapons drawn, he knew what they were up to and struck first.
In The Cowboys, Bruce Dern's character shot John Wayne in the back, and was typecast for life as a villain.
In Real Life, poor Dern received death threats from fans upset that Wayne's character was killed like that.
In White Sun of the Desert, Said reveals Off Stage Villain Djavded killed his father from behind. Said's current apparel has several bullet holes in the back - but whether he inherited it from his father or liberated it from someone else is unknown.
In Gangs of New York, Bill the Butcher throws a meat cleaver into Monk's back after he is elected sheriff, before beating Monk to death wih his own shillelagh.
In a famous scene in The French Connection Popeye Doyle shoots a man in the back. The real Popeye objected to this portrayal at first but eventually learned to accept it under the Rule of Cool.
In White Heat, when Arthur Cody Jarret learns that his mother was dispatched through being shot in the back, it's strongly implied that his plans for vengeance against the backshooter in question became all the more imminent and brutal
In High Noon, Amy shoots Pierce, one of Miller's men, in the back through a window while he's trying to reload.
An interesting inversion happened in John Wayne's final film, The Shootist. In the original script, Wayne's character was to shoot someone in the back during the final gunfight, but Wayne refused to do so, saying, "...I've made over 250 pictures and have never shot a guy in the back. Change it." Despite protests from Don Siegel, who insisted that good friend Clint Eastwood would've shot a guy in the back, the script was changed, and the final shootout played to Wayne's satisfaction.
Who, in an Ironic Echo, himself is shot in the back by Megatron. He doesn't die but is cripped enough for Optimus to finish him off with a headshot right after decapitating Megatron in the space of 5 seconds... while missing an arm.
Nobody realises ahead of time that someone intends to shoot him in the back and sets up a humiliation for them when they try.
In the Alternate Timeline of Stargate Continuum, Ba'al has become the Supreme System Lord and has taken Qetesh (in Vala's body) as his bride. However, when Qetesh sees that Ba'al is not being as "Goa'uldey" as he should (he doesn't want to destroy Earth out of pragmatism), she stabs him in the back with a monomolecular blade. She tries to get Teal'c cooperation, but he retorts that she'll kill Ba'al anyway. She does.
There are other cases in Stargate SG-1 itself, including the good guys. For example, Teal'c android copy shoots Cronus in the back with a staff weapon in revenge for killing Teal'c father.
In Push, when Kira pushes Agent Mack to think his partner Agent Holden killed his brother (Mack doesn't have a brother), Mack walks out of the bathroom and shoots Holden in the back before realizing what he'd done.
Skyfall. James Bond kills cyber-terrorist Big Bad with a knife thrown into his back as he's about to kill M. This has Rule of Symbolism given the film's theme of whether assassins like Bond are obsolete in the modern era, not to mention the general theme of betrayal.
Kit's father in The Phantom was killed by being stabbed in the back by Quill.
The Crow. During Eric and Top Dollar's battle on the church rooftops, Eric is momentarily distracted by a distressed Sarah. Top Dollar takes a cheap shot by sneaking up behind him and stabbing Eric through the back with his sword.
This is how the kaiju Leatherback destroys Cherno Alpha in Pacific Rim; by leaping onto Cherno's back, dragging it underwater and smashing it to pieces.
Scream: Given the films' subject matter, and Ghostface's uncanny ability to pop in and out, this happens a lot. How fatal this proves to the character on the receiving end depends on if they've already been injured in some other way, if they're an important character, or if their name happens to be Dewey. Jarring when you consider the cases of Dewey and Kate Roberts; they both get very similar injuries (albeit in different films) in the same place, yet Dewey survives hours without medical attention, and Kate dies mere seconds after the stabbing occurs.
Subverted in Escape from L.A.. A Neo-Nazi skinhead attempts to throw a knife into Snake's back as he's walking away. Snake turns around and riddles him with bullets before continuing on his path.
In the Nibelungenlied, Hagen spears Siegfried in the back while Siegfried is drinking water from a spring during a hunt. This also happens in most other versions of the Siegfried story. (Siegfried's one vulnerable spot happened to be on his back, by the way).
Which, if you think about it, is not so much cowardly as it is reasonable, unless you think your enemies ought to be obligated to attack you only in ways that cannot possibly harm you.
The Takers by Jerry Ahern. When he's trapped on a yacht that's been hijacked by pirates, the hero Josh Culhane has to do this. As he's a writer of action novels where the hero isn't supposed to shoot people in the back it bothers him, but not enough to stop him from doing it.
He also shot Captain Luccio in the back of the head. In his defense, she was possessed by a psychotically evil Omnicidal Maniac necromancer who would have just possessed him if it had realized he'd figured it out.
In Small Favor, Harry threatens a thug with this if he tries to run, pointing out it could mean his spine got hurt.
Do turn around, please. I want to look at you, for I never like shooting anyone in the back.
Lampshaded in The Golden Gate by Alistair MacLean, when the reporter is not happy to find out that the secret agent is armed with poison dart weapons: "to be shot into the back, I suppose." The hero calmly points out that a dead man doesn't care from what direction he gets shot.
Inquisitor Eisenhorn has the courtesy to wait until the enemy is facing him. Though he still shoves his pistol in Eyeclone's mouth and fires while the renegade isn't expecting to spot him. Just imagine the Oh Crap look on Eyeclone's face!
And during our introduction to Bequin she saves Eisenhorn by stabbing a thug through the back of the neck.
In Nick Kyme's novel Salamander, the Salamander captain is shot in the back. At the climax, Dak'ir accuses the killer of having no honor because of it, and a renegade Space Marine such as he defends himself; he had to obey orders.
In Cold Mountain, a minor character is terrified of being shot in the back, which would prove he was fleeing the battle. He is eventually shot in the chest, whilst running backwards.
In C. S. Goto's Blood Ravens novel Dawn of War: Ascension, at the Blood Rites, aspirants are stabbed In the Back by others whom they had trusted to guard their backs.
In Chris Roberson's Imperial Fists novel Sons of Dorn, Jean-Robur learns to fight like a Combat Pragmatist in his first battle, stabbing a foe In the Back.
The Casino Murder Case: The murderer is holding Philo Vance and district attorney John Markham at gunpoint. After shooting Vance, he tells Markham that since Markham is "only a cop at heart", he would shoot Markham in the back. Markham refuses to turn around. Vance was Playing Possum after switching the murderer's bullets for blanks, hoping The Cavalry, a.k.a. the NYPD, would kill the murderer and save everyone the hassle of a trial. It worked.
Happens in one of the Redwall books, because the shrew in question is a Jerkass and a coward that throws his rapier at Log-a-log, killing him.
In Götterdämmerung, Hagen stabs Siegfried in the back in a more elaborate version of the scene from the Nibelungenlied. In the opera, Brünhild had cast a spell that made Siegfried invulnerable from the front because she said she knew he would never turn his back on an enemy.
Star Wars: The Old Republic: Revan has three main characters: Revan (redeemed from the Dark Side), Meetra Surik (the Player Character from KotOR II), and Darth Scourge (who realizes that The Emperor is planning to start a war the Sith Empire can't win and wants to stop him). At the end of the novel, the three characters have joined forces and are facing down the Emperor. Scourge has a Force Vision that shows him a different Jedi killing the Emperor. Realizing they can't win, he stabs Meetra in the back, killing her instantly, while the Emperor disables Revan.
The Greyhawk novel White Plume Mountain has this as one of The Justicar's main points:
Justicar: If they can hit you back, you're doing something wrong.
First Lensman opens with the visitor to a lab sneaking up on a scientist with a handgun and shooting him in the back seven times, including twice in the head — only for every bullet to pass through him harmlessly. Coupled with the scientist's blasé reaction and addressing the shooter as "Gharlane of Eddore," this is how we find out the shooter and the scientist are an Eddorian and an Arisian (respectively) in disguise. (No one notices any of this thanks to their zones of compulsion.)
Corelle Whistler shoots someone in the back in A Brother's Price. Aware of this trope and unwilling to look like a traitor or a coward, she justifies it, noting that the woman she shot was trying to kill Eldest.
In the Paladin of Shadows book Unto the Breach a Keldara says that he doesn't like slaughtering people from ambush, even if it is very effective.
On Supernatural Sam is killed the first time by a stab in the back. Subsequent deaths are much less dramatic, until three seasons later he jumps into Hell to save the world, and technically doesn't die.
In the Season 6 finale, Sam attempts to kill Castiel, who has just declared himself God with a stab to the back. It doesn't work.
Also in the Season 6 finale, Castiel kills Balthazar with a back stab.
Garak on Star Trek: Deep Space Nine had no qualms about this. Being pragmatic is the Cardassian way, especially if you've received Training from Hell meant to prepare you for the life of an operative. In "Call to Arms" he notes to Odo that when he and Gul Dukat were fighting to keep the Klingons away from the Detapa Council in "The Way of the Warrior" he had an opportunity to shoot Dukat in the back for just a moment.
Odo: You'd shoot a man in the back? Garak: It's the safest way, isn't it? But then I thought, "oh, no, I can't fight all these Klingons by myself." So I let him live. Odo: And now you regret it. Garak: Ah, my dear Constable, before this day is over everyone on this station is going to regret it.
In "The Changing Face of Evil", Gul Dukat disguised as a Bajoran convinces Kai Winn to align towards the Pa-Wraiths. Winn's assistant finds this out and a conflicted Winn stabs the assistant in the back.
[Smith tries to shoot one of the aliens in the back. The aliens make the laser pistol disappear] Alien: You showed treachery and cunning, noble qualities on your native planet no doubt, but indicating defective relays in your reasoning circuits.
Chris Larabee: You don't shoot nobody in the back!
Angie: Aww, what you gonna do? Shoot me in the back? That was always the difference between you and me. See, in your place, I could pull that trigger.
Mal's final conversation with Simon in the pilot of Firefly has these words assuring Simon that he won't kill him in his sleep: "You don't know me, son, so let me put this to you plainly. If I ever kill you, you will be awake, you will be facing me, and you will be armed." Given Mal's later nature as a Combat Pragmatist, this seems more an acceptance of Simon and his sister into Mal's True Companions than anything else. Joss Whedon suggests sardonically that Mal just has a very particular code of honor that will let him shoot an unarmed, surrendering enemy...but only in the front.
Alternatively, the monologue could be taken to mean that Mal just plain isn't going to try and kill Simon, and would only do so if Simon was trying to kill him.
A bit of Fridge Brilliance here, Dobson was awake, armed and looking at Mal when he was killed. The first time anyways.
After Jayne betrays Simon and River for money, Mal calls him out on it:
In the season 4 finale, Keamy criticizes Richard for shooting him in the back (which also wasn't effective because Keamy was heavily armored.) Richard hadn't had a choice, though, as Keamy was grappling with Sayid at the time.
In "The Variable," Eloise shoots Daniel, her own son, in the back.
SCTV - an ad for the tv Western "Yellowbelly" shows just what a yellowbelly the title character (John Candy) is as he shoots a child in the back, and then does the same to the child's mother.
Horatio Hornblower is nearly stabbed in the back in the mini-series, by a furious and humiliated Simpson, but is saved by a truly excellent shot by Captain Pellew.
This is where Jimmy is shot in Degrassi, causing him to be paralyzed from the waist down for the remainder of his time on the program. This is also where JT is fatally stabbed.
In the Criminal Minds episode "Masterpiece," Rossi stays pretty composed while the unsub of the week tries and fails to kill his team and destroy him in various other ways. He loses it, however, when that same unsub attacks him from behind.
Though even though the Mad King was obviously completely unfit to rule and a danger to the entire kingdom, Jaime was called "Kingslayer" from then on and was met with disdain from Lawful Good (or Lawful Stupid, if you prefer) characters like Ned Stark, who believed that what he did was dishonorable. This despite the fact that he did it to prevent him from burning down and killing everyone in King's Landing.
Lampshaded in the TV series. "Tell me; if I'd stabbed the Mad King in the belly instead of the back, would you admire me more?"
Inverted when Fiery Redhead Ygritte puts three arrows into Jon Snow (two in the back and one in the thigh) as he rides off to warn the Night's Watch of the impending wildling attack. It's actually Jon who betrayed her, but he thinks she won't shoot because they love each other. As Ygritte is fond of pointing out, he knows nothing.
Happens in CSI Ny,in the season finale where the bar the CSIs are sitting in is shot to bits by a suspect wanting revenge. However, it's mainly a result of the villain shooting anywhere and everywhere with some kind of automatic weapon and Danny, who was sitting closest to the window anyway, having thrown himself over Lindsay to protect his wife, leaving his back exposed.
Kaizoku Sentai Gokaiger - Damaras forces Basco into an alliance against the heroes to avenge Warz. Basco betrays Damaras the very next episode, and appears out of nowhere to literally backstab him the second he realizes.
The first mission in Mass Effect 1 starts with Nihlus, a turian Spectre, getting shot in the back by his old colleague Saren.
In Mass Effect 3, after being soundly beaten by Shepard, Kai Leng limps up to them as they're perusing through the Illusive Man's files and attempts to stab them in the back. It doesn't work.
Shepard can do this with more success earlier in the game, but the event is widely considered a Moral Event Horizon by players. If choosing to sabotage the Genophage Cure while Wrex and/or Eve are still alive, the player shoots Mordin in the back to try and stop him from curing the krogan. You're then forced to watch Mordin struggle on and almost make it to the cure console in a scene custom-made to make you feel like a monster for doing it. If Wrex is alive when you choose this option, you're going to have a much more face-to-face confrontation later.
Mobster1: [back turned to the player] Always figured someone would sneak up behind me in the middle of the night and shoot me in the back.
In Fallout 3, one of the first quests is to decide if you want to set off or defuse an atomic bomb in the middle of a town. If you tell the town's sheriff about the individual who offers to pay you to set it off he will go to confront him and be shot in the back for his trouble (unless you save him, but this doesn't effect the game much).
A rare good guy version occurs in Quest for Glory III; when the five heroes enter the Demon Wizard's inner sanctum, they're confronted by super-strong, demonic versions of themselves created via magic mirror. The player's fight seems unwinnable until the thief from the bazaar appears and backstabs the evil version, telling the hero to go on and face the wizard while he holds off the copy.
In Super Smash Bros.. Brawl: The Subspace Emissary, Ganondorf shoots Bowser with the Dark Cannon from behind..
The Back Story of Alexandros Mograine features this. His son Renault was persuaded to lead him into a trap — a hopeless fight against wave upon wave of undead. Because he was such a Badass, he killed them all — but when he took a breather afterward, and let go of his sword, Renault came out of hiding and stabbed him with it.
Orgrim Doomhammer died when he was stabbed in the back with a polearm. Those who look up to him consider this a cowardly act.
One quest in the Twilight Highlands involves fighting some enemies long enough for your ally to sneak up on them and finish them off.
The Kor'Kron Assassins in Siege of Orgrimmar can backstab people who turn their backs to them.
Interestingly enough, in the Fallen Protectors encounter, He Softfoot uses a Gouge attack on tanks; if the tank is facing him when it hits, the tank will be stunned and Rook will fixate on a random member for a few seconds. If the player is facing away, however, the attack will merely knock them back.
It's generally advantageous to attack an enemy from behind when you can, since they can't parry attacks from behind or hit you with frontal cone abilities, and some enemies are completely immune to frontal attacks. Dragons and other similar creatures are the exception, since they can tail lash people who attack from behind, so the only solution is to attack from the side.
At the end of Conquests Of Camelot, the thief that stole the grail tries to stab King Arthur in the back, which fails since he's wearing armor (He is not a moron). The guy is then promptly reduced to a skeleton by the grail's power for his bad manners.
Tales of Monkey Island: Inverted near the end of Chapter 4: Not only does LeChuck (figuratively) stab Guybrush in the back in a clever ambush attack, but he (literally) stabs Guybrush in the chest... with the Cursed Cutlass of Kaflu that is powerful enough to kill him, if not instantly!
In Kingdom Hearts: Birth by Sleep, on Terra's route, after the boss fight with Master Eraqus, both have a My God, What Have I Done? moment, until Master Xenanort stabs Eraqus in the back (while Terra watches), killing him (although in Blank Points, it's shown that Eraqus' heart went into Terra's).
Later on in Super Robot Wars Z3: Jigoku-Hen, when Char does decide to go to the frontlines, he confronts Amuro face-to-face. But Full Frontal is such a jerk that while Char and Amuro are talking, he attacks Amuro from behind which makes Char pretty angry.
This is how the Nanaya clan in Tsukihime operates. As they're almost entirely normal humans going up against demons and vampires, the only way to win is to get close to them before they notice you and cut them to bits. Kiri, Shiki's father, actually used a mace to crush skulls, throats and internal organs. Shiki can mostly avoid this one because his eyes cheat, though he does take out at least one DAA like this in supplementary material.
In the earlier editions of Dungeons & Dragons, the thief did his best damage from behind — he got an attack bonus for attacking an unaware opponent from behind, and if he hit, he did double damage (or even more depending on his level). It was later reworked into the "Sneak Attack" rules of third and fourth edition.
Non-Skirmisher units in Warhammer Fantasy get massive bonuses to combat resolution (the check to see if the enemy breaks and runs) if they charge from the side or rear.
While creating the Alara shard of Bant in Magic: The Gathering, a world where the orderly, honorable and noble white mana ruled without any influence of the selfish corruption of black or chaotic brutality of red, one member of the team simply thought of the idea "They don't wear armour on their backs". Yes, in this world, the thought of someone being such a dirty coward that they would attack an enemy from behind is so alien to them that they don't even consider it.
In The Fantasy Trip, attacking an opponent from behind gives you a significant DX bonus, which means you're much more likely to hit. There is no direct damage bonus, but the DX bonus might make it feasible to take an aimed shot, which stands a significant chance of disabling or killing the target, depending on what you're aiming at and how much damage you do.
Sarine in Errant Story has the dubious honour of being the first to backstab someone from the front with a sword.
Bugs Bunny: Only a rat would shoot a guy... (turns around) ...in the back. Elmer: (starts to pull on the trigger) Bugs Bunny: I reiterate: only a big, fat rat would shoot a guy in the back. Elmer: (shoots — a smoke cloud appears where Bugs was) Elmer: (gloating) So I'm a big, fat wat! Bugs Bunny: (appears out of smoke) Ah! Have some cheese, rat! (shoves cheese wedge in Elmer's mouth)
In G.I. Joe: Renegades, The Baroness isn't having much luck against Snake Eyes, so when he turns to face Storm Shadow, she takes the opportunity to get in a free hit (and earns a very threatening reproach from Storm Shadow).
Wild Bill Hickok was shot in the back by Jack McCall. It happened to him the only time he ever sat with his back to the door.
McCall shot Hickok in the back of the head, whereupon Bill stood up and began to draw his pistol and turn toward his attacker before falling across the table dead. Reflexes like that qualify Hickok for Epic Badass status.
Famed Lakota warrior chief Crazy Horse was reputedly bayoneted in the back.
Famed gunslinger and killer John Wesley Hardin was shot in the back. His killer got off by claiming self-defense on the grounds that Hardin could see him reflected in the barroom mirror Hardin was looking at.
While in the Wild East... One 19th Century Chechnyan warlord surrendered to Russians (yes, Chechen Wars aren't exactly new stuff), and the other kept fighting. On one occasion they met, and when the first one was leaving, the second shouted at him to turn around, because he wanted to at least shoot him (for mentioning surrendering to Russians, what he saw as treason), but wouldn't shoot him in the back.
Brutally true in Real Life. It has actually been proven that it is significantly easier for soldiers to kill their opponents when their backs are turned, especially if they are retreating. An opponent facing a shooter or attacker is a human being and a face that conveys a variety of emotions, immediately triggering the fundamental human aversion to killing one's own kind. On the other hand, a person fleeing triggers predatory "pursue and kill" instincts, and the killer doesn't see their face, thus dehumanizing their target, making shooting or stabbing them much easier. A person from behind also resembles the standard "person silhouette" target used on many target ranges, making it easier to revert back to training mentality and shoot.