Happened to Sailor Mnemosyne and Sailor Lethe in the Sailor Moon manga. They were this close to a Heel-Face Turn when Mnemosyne Usagi's memories, which were deleted by Sailor Lethe, who was also willing to make a Heel-Face Turn along with Mnemosyne after the restoration, but then they got killed off by their evil superior officers Chi and Pi.
Likewise, Sailor Tin Nyanko was partially purified by Sailor Moon in the anime. Galaxia doesn't give Sailor Moon the chance to finish by killing her off herself.
Nephrite in the first season of the anime dies immediately after leaving Beryl's organization. Whether this is this trope or Heel-Face Turn + Redemption Equals Death is a real judgment call since Nephrite dies before having a real chance to turn against Beryl or do any substantive damage to her, but does at least manage to save Naru's life, and did so rather than kill Sailor Moon even though he had her right where he wanted her.
It's worse for Kunzite in the manga who recovers his old memories of being Venus' lover in their past life but realizes he cannot betray Beryl because she controls Endymion. He's forced to fight against the woman he loves until she kills him. Made more evident in the musicals where he confesses to her just before she kills them BOTH with Crescent Beam.
In Bleach, Tosen is defeated and near death, and actually seems to be leaning toward repenting in his final moments. Then Aizen kills him.
There are a lot of these in Bleach. Gin realizes that he's spent his whole life playing the villain and was unable to kill Aizen or make Rangiku happy, Ulquiorra starts to recognize emotions just as he's disintegrating, Halibel turns on Aizen after she realizes he's just been throwing their lives away and is promptly killed for her troubles (though she turns out to be Not Quite Dead later), and Nnoitra casts a debatably meaningful glance at Nel, whom he tried to kill more than once basically just because he's a misogynist with an inferiority complex (though there's some indication that his true desire had been to provoke her into killing him).
In Pokémon, on the rare occasions Team Rocket attempt to go straight, Team Twerp blast them into the sky anyway, sometimes completely by accident.
Shinsen Tennozu of Speed Grapher is the head of both a legitimate multi-billion dollar organization and the shadier criminal group that helped make it so powerful. Also, she's aterriblemother. She gets a very Karmic Death at the hands of her Magnificent Bastard right-hand man, and as she's dying on the floor her daughter Kagura comes running up to her and tearfully reveals that Shinsen's lover (Kagura's father) hadn't abandoned her: he had been unavoidably delayed for their rendezvous and had subsequently been murdered by the secret organization he was working with. The look on Shinsen's face suggests that she truly regretted allowing herself to become the bitter and hateful person that she had, but it was too late to even try to make up for it and she died moments later.
Chirin No Suzu has this happen to Chirin at the very end. He kills the Wolf, but the sheep throw him out. Chirin is left with nothing. This is one of the contributing factors to the tragedy of the story.
Afro Samurai offers a non-death variant. At the very start of his journey, Afro's obsession with revenge resulted in the death of all of his friends and his teacher. This is the one of two times in the series he actually show regret for his actions, but he quickly accepts afterwards that there was no turning back at that point.
In Muhyo And Roji, Kid Cortlaw turns against Buhpu and leads to his defeat after learning that he killed Ivy and lied that the MLS did it. He realizes that his allying with Ark to bring back his parents was wrong, but is unable to do anything more before he dies.
Yamada gets this in Highschool of the Dead when he starts to show even the least bit of concern for his family. The entire Orgybus votes to murder him on the spot (read: leave him to the zombies). In the anime adaptation, Misuzu may have regretted killing Toshimi just as the zombie mob behind her crushed her own head.
Mirai Nikki: Yukkii's father was an all around horrible parent who not only tries to break Yukkii's phone so his debt would get paid off, but also leaves his son to die, and kills his own ex-wife so she won't turn him in to the police. Still, he does realize that what he did was wrong and was going to make it up to him by turning himself in and finally going to look at the stars with his son...that is, until he gets stabbed in the gut immediately after he declares this.
Naruto: After Obito / Tobi has the tailed beasts extracted from him, he, Kakashi and Edo! Minato discuss what should happen to him next. Obito chooses to copy Pain and sacrifice himself reviving his victims. Zetsu interrupts, hijacks his body and forces him to use the resurrection technique on Madara.
Legion of Super-Heroes: V4 has a wretchedly sadistic example in Sun Boy, a long-standing Legionnaire who'd been seduced into being the public face of the evil Earthgov. Faced with an impending disaster, he tried to put his costume back on and save the city, only to a) meet with public scorn and b) be caught in the explosion of a fusion reactor. Wait, we're not done: due to his powers, the explosion didn't kill him (actually, due to his powers the explosion shouldn't even have touched him; oh well); instead it left him a mangled, screaming, burning wreck. He then spent the next year and a half on life-support, in agonizing pain and reliving his worst memories, until finally he was shot by his lover/handler (who then killed herself). That door slammed so hard it broke.
In Nikolai Dante, during the Battle of St Petersburg, a random mook suddenly had a change of heart and shoved a machine gun operator out of the way to stop him firing on the Romanov forces. Immediately afterwards, Jena, unaware of what he had just done, stabbed him in the chest.
In Spider-Man Mac Gargan, a.k.a. the Scorpion, ends up wandering the sewers at one point, depressed but with a clearer head than he had had for a long time. Eventually, his sanity starts to return, and he is about to make a Heel-Face Turn when Spider-Man having depression issues of his own, finds Gargan and attacks him savagely ignoring Gargan's pleas that he had changed. Gargan then goes crazy again to save himself and continues his life of super-villain debauchery, first as Scorpion, and later as a new Venom.
Another example involving a Spider-Man villain was Shriek, one member of Carnage's group of killers during the Maximum Carnage fiasco. Some months after that fiasco, she broke out of Ravencroft Asylum and kidnapped Malcolm McBride (formerly Carrion, who was human at the moment, the virus inside him in remission) and stress caused by the ordeal caused him to become Carrion again. After seeing Carrion show some sympathy for his mother, Shriek had a brief moment of sanity, and actually tried to absorb the Carrion Virus into herself, trying to make a Heroic Sacrifice to save Malcolm. She did save Malcolm, but the virus didn't kill her... And for poor Shriek, this heroic act only made it worse for her. It drove her deeper into madness, making her think the virus inside her was an unborn child, and she spent her time in her cell eagerly anticipating the birth of her "baby" like an expectant mother. Eventually, the Jackal assaulted her to get the virus back, and when next seen, she was back with Carnage and worse than ever.
A Star Wars Tales comic tells the POV account of the first stormtrooper to rush into the Tantive IV in A New Hope. Desperate to leave his backwater, dead-end planet, he murders a subversive man as a loyalty test for an Imperial officer and leaves with them to become a stormtrooper. He has witnessed horrors, committed unspeakable atrocities, and, all this time later, is beginning to lose his cold, ruthless demeanor. He wonders if he hadn't made the wrong decisions, and begins to consider jumping ship and finding the Rebel Alliance. Then Princess Leia shoots him in the face.
In Ultimate Spiderman, Doctor Octopus refuses to help Norman Osbourne kill Spiderman. He explains that he is sick of being evil and wants to make a fresh start for himself. He also tries to convince Osbourne not to kill Peter, since Spiderman is their greatest "creation". Norman won't have any of that and kills Otto after a vicious fight.
Usagi Yojimbo features a story involving a crooked bookie who has employed a skilled former samurai to help him run a con; they hustle wandering travelers into fighting duels with the samurai, the bookie takes bets on the fight, hyping up the unskilled traveler as unbeatable, then they split the pot when the samurai kills the traveler. The samurai begins to feel bad about this dishonest life, however, and tells the bookie he's quitting. The bookie forces him to fight one last duel to pay off a debt to him, and said duel just happens to be against Usagi, who knows nothing of the con or of the samurai's intention to turn over a new leaf. They fight the duel, and Usagi wins easily, killing the samurai. The last page of the story shows the dead samurai's wife and son, unknowingly waiting for him to come home...
Though how much deciding that you're only going to murder one more person for money counts as a Heel-Face Turn is a matter of some dispute.
The samurai doesn't appear to feel remorseful and the bookie doesn't appear to force the samurai into one last fight. The samurai wants to earn enough money to care for his family and the fight with Usagi just happens to be the one that would give the samurai enough Ryo to live a quiet, peaceful life with his wife and child. The samurai fights this last fight not because the bookie is forcing him to, but because he wants more Ryo.
In V for Vendetta, the head is shot down by the time he began to ponder about his ways and the possibility to change them.
Deadpool tries really, really hard to be good. He does. Really. But it's just so much easier to kill people and he has no attention span and people are SO annoying and...
That's not to say he's a Heel, but rather that despite all his attempts to be good, and all his actually good acts, he's still a nutcase killer-for-hire.
Well, Fearless Leader's call does interrupt them during the above mentioned conversation, and during the Final Battle they do keep fighting on his side. So, it's more like they were considering giving up evil, but ultimately decided "The Hell with it" and slammed the door shut themselves.
Boris was actually glad Fearless Leader interrupted the conversation before he had to tell Natasha what he thought about her idea of the both of them starting a family.
Kung Fu Panda 2 has the Wolf Boss refuse to fire cannons on his own men with a firm 'No', only to be immediately killed by Lord Shen's throwing knives to the throat.
Films — Live-Action
The pivotal scene of Sméagol's near-repentance on the stairs was omitted from the film Return Of The King due to the director feeling it necessary to maintain that Sméagol had never been redeemablein the first place. This decision to elevate Gollum from antagonist to outright villain was challenged by the writers, so the scene was replaced with another climactic episode in which Frodo spares Gollum and Gollum almost repents, but Frodo then admits the purpose of the quest to Gollum, at which he snaps and attacks Frodo again.
At the beginning of the third movie (in an extended scene), it looks like Wormtongue is finally about to abandon Saruman and join the heroes, giving them insight on the enemy's plan. When Saruman realizes this, he strikes him across the face. Wormtongue is so upset by this that he stabs Saruman in the back and is shot down by Legolas, resulting in yet another point against diplomacy in Middle Earth.
Though this is EXACTLY how Saruman and Wormtongue die in the books, albeit significantly later in the time-line. Wormtongue is shot by the hobbits, who assume (probably correctly) that he's gone completely mad. The heroes (just the hobbits by this point) offer to allow Wormtongue to stay in the Shire even as they're throwing Saruman out, but Saruman himself closes this door:
Saruman: Worm killed [Lotho Sackville-Baggins]. Stabbed him in his sleep, I believe. Buried him, I hope, though Worm has been very hungry lately. No, Worm is not really nice. You had better leave him to me.
In Shaun of the Dead, David has been a dick towards everyone (but especially Shaun) pretty much throughout the entire movie, largely due to his unrequited love for Shaun's girlfriend Liz. This culminating in David forcing Shaun to shoot his own mother (not without cause, since she was a newly risen zombie, but his insensitive and smug way of handling it didn't help matters) and then trying to shoot Shaun himself. After being called out by his own girlfriend about his actions, David is on the cusp of sincerely apologising to Shaun for all he's done... when an army of zombies break through the window behind him, drag him out, tear him to pieces and eat him before he can actually say it. Ouch.
Gordon's blunt dismissal of Maroni's indignation at the murder of Rachel Dawes by The Joker's minions in The Dark Knight. Subverted when Maroni goes ahead with ratting the clown anyway ("You want him, I can tell you where he'll be this afternoon") and later reveals to Harvey Two-Face the name of the other traitor in Gordon's unit, then ultimately left up in the air when Two-Face kills his driver.
Played straight in Batman Begins when Joe Chill, who agrees to testify against Carmine Falcone in exchange for early parole, is assassinated by one of Falcone's goons immediately after the hearing.
Morrow's character was supposed to be redeemed after risking himself to save two Vietnamese children in a rice paddy. It was during the filming of this scene where the freak accident occured that claimed the lives of Vic Morrow and the two child actors, and the real footage of the accident still exists.
In Austin Powers: International Man of Mystery (the first movie), Number 2 pulls a gun on Dr. Evil, decrying his old-fashioned villainy and pointing out how he made Virtucon disgustingly rich through completely legal means. He starts to negotiate a surrender with Austin Powers, only to sit in the wrong chair...
He returns in the next film, still on Dr. Evil's side and with just some slight burns on his face.
The 1949 version of The Great Gatsby plays this to the hilt. In the climactic poolside scene, Gatsby tells Nick that he plans to turn himself in and take the rap for what he has (and hasn't) done, and go straight. The Door Slams him immediately afterward.
The ending of American History X involves a particularly memorable one; the day after Danny leaves the Neo-Nazi movement, he is killed because he pissed off a bully (who happened to be a minority) he had a feud with (and for further irony, was probably pressured into it the same way Danny was pressured into Nazism and then out again by his own Big Brother Mentor). Some alternative endings have this causing Derek to revert to Nazism, undoing all the Character Development of the film.
In Four Lions one of the protagonists panicked and no longer wanted to be a suicide bomber, so he tried to turn himself in to the police. While he was yelling about how his costume was filled with explosives but he wasn't going to blow himself up, one of his friends blew him up to stop him giving the rest of them away.
The Patriot has two instances where it shows British officers being morally conflicted about their actions, but nothing is done with it:
First up is a lieutenant. The movie seems to make rather a big deal about him reacting with horror and distress at Colonel Tavington shooting Benjamin Martin's son Thomas and killing the wounded Militiamen. There is even a shot of him reacting to the sight of one of the younger boys crying before departing. But then the movie just kills him off in the very next scene.
Next we have Captain Wilkins, a Loyalist officer who joins the Green Dragoons and whose zeal leads him to boast that traitors to the crown deserve to die. However then Tavington tells him to burn a bunch of townsfolk in a church, he protests this as being dishonorable and murderous. Nevertheless Wilkins readily caves when Tavington bullies him into it, essentially shutting the door on his own redemption himself. Despite this, there still seems to be some hope for him, given how visibly upset he is watching the church burn, however nothing is ever done with him and apart from a few snarky comments aimed at Tavington, he more or less vanishes from the movie entirely in the final battle (this troper can't even remember if he died or not).
We get a version of this in Skyfall, when Silva has M cornered and his gun in her face, when he realizes she's already mortally wounded and will die no matter what he does. He breaks down, puts the gun in her hand, raises it up to one side of her head while pressing his own head to the other side, and begs her to kill both of them with the same bullet. Then Bond barges in and throws a knife into his back. Silva gets demonstrably annoyed at this before keeling over.
Stephen King's It had Victor Criss, the only one of the three bullies who seemed redeemable. At one point he warns the Losers of Henry's deteriorating insanity, and later on he sincerely considers joining up with them instead. Ultimately he goes with Henry out of loyalty and ends up killed by the eponymous monster.
In Astrid Lindgren's Kid Detective story series Kalle Blomqvist, a Swedish inventor and his preteen son are kidnapped by (ostensibly) Soviet agents. However, one of them soon starts to get qualms about having to take the son away as well, and even tries to help the kid detectives. He gets a bullet in the back for his troubles, but ends up in hospital and lives.
In The Lord of the Rings, Sméagol's literal face turn on the stairs of Cirith Ungol. Sméagol's gentle touch of Frodo's knee is misinterpreted by Sam as an attack on his master (or molestation, as Sam's wording of it implies.) This marks the last point in the book where Sméagol had a chance at redemption...note But Frodo's "faith" in Gollum is rewarded when Gollum ends up destroying the Ring anyway, and Sam also makes up for his earlier mistake by sparing Gollum when he has a chance to kill him, which helps result in said destruction of the Ring.
Wormtongue appears to have one of these in the penultimate chapter. You can sense that he really does want to leave Saruman, but as he's hesitating, Saruman mocks him. This drives him to kill Saruman, and then the hobbits kill him.
In The Stand, an especially ironic example is the story of Nadine Cross, who spent her whole life believing she had to save her virginity for Randall Flagg, who is revealed to be The Antichrist; when she falls in love with good guy Larry Underwood she initially rebuffs his advances, then desperately asks him to sleep with her to remove the spell, after he has fallen in love with someone else. When he refuses, Nadine falls in with Flagg.
In The Wheel of Time, according to Word Of God, Asmodean was genuinely trying to switch sides—if only for lack of options—when he was killed.
Of the less lethal variety we have Zaknafein from the Drizzt books, who realized the evil in his society but was unable to escape it or to fight against it and became resigned to being the least evil he could get away with.
In the Coldfire Trilogy, the Hunter, finally no longer undead and free to try to redeem himself, is promptly killed. Sort of.
Jacen Solo from the Star Wars Expanded Universe is an Aversion - while he had realized that he'd made a number of mistakes, he had no intention of turning back. It's just that what he was trying to do at the time of his death was a good thing.
Before it even happened in Prisoner of Azkaban, Dumbledore states that Harry sparing Pettigrew's life would give Pettigrew a chance to do the same later on. Seems Voldemort was on the same page and intentionally cursed Pettigrew's Evil Hand to kill him if even considered betraying him.
Another possible example from the book would be the fate of Regulus Black, a former Death Eater who sacrificed his life in an attempt to destroy Voldemort'sHorcrux after realizing just how bad Voldemort was. Unfortunately, not only did Voldemort have more than the one Horcrux, but his accomplice was unable to destroy the one he did get - and because no one knew what he'd done, Dumbledore still ended up being horribly wounded pointlessly trying to retrieve it from the original hiding place.
Barty Crouch Sr. in Goblet of Fire. His crime was helping his Death Eater son escape from Azkaban and keeping him in hiding for years, until Voldemort came. Crouch escapes Voldemort and heads to Hogwarts to confess to Dumbledore, but is killed by his son on arrival.
In a true horror instance, the God of the Left Behind books continues his series-long progression across the Moral Event Horizon by rendering people who've accepted the Mark of the Beast unable to repent even though they desperately want to. At one point a character tries to recite a prayer, any prayer, and can only recall the first line of "Now I lay me down to sleep".
This is at least semi-Scriptural. According to Revelation, there are some decisions you can't take back. It's not clear from the Scriptures, however, whether that means you aren't allowed to repent, or whether it's just that nobody who goes that route ever will sincerely repent.
Leon Fortunato, when he is brought before Jesus along with Nicolae Carpathia, tries to repent and gets the Heel Face Door Slam treatment from Jesus since He considers Leon long since sold out to Satan.
Vermin in the Redwall universe can never repent, ever. Outcast of Redwall was a particularly strong example, where a ferret raised in the abbey turns out evil, just because of his nature as a ferret.
But there have been a couple cases of vermin turning out good. In The Bellmaker, for example, the former pirate Blaggut ends up leading a life of peace at the end of the book. Most vermin with streaks of good realize it a bit late, however.
Then again, there are also many cases of antagonists not outright joining the good side, but abandon or otherwise inconvenience the bad side, and live to see the end. In the second book, for instance, The SmartLancer at the side of the Big Bad runs off into exile when she gets too dangerous for him.
The Romulan Star Empire has one of these in the Star Trek: Vulcan's Soul trilogy. Praetor Neral is about to lead the Romulans into formalizing and strengthening alliance with the Federation (as well as lift many of the restrictions placed on the Vulcan unificationist movement). Neral has come to realize the Federation can be trusted and that they are worthy friends to Romulus...but he's unable to implement any of his new policies before he's killed off, replaced by conservative leaders who take Romulus back into hostile isolationism.
Glen Cook wrote three short stories about a crew of pirates under a Flying Dutchman sort of curse. They realize in the second story that any of them who do something genuinely good can escape their eternal wandering, and the ship's captain sacrifices himself to destroy an even worse evil — leaving the narrator as the new captain, wondering what he did wrong that he wasn't freed as well...
The In Death series: In Ceremony in Death, one lawyer who happens to be a Satanic cult member gets evidence shoved into his face that one of his fellow cult members has been murdered. Later, he realizes that he's been having blackouts ever since he joined the cult...and that he witnessed that one cult member murdered in a sacrifice in one of those blackouts! He starts wonder what else happened in those blackouts. He ends up deciding that he can't defend the cult leader anymore and that he should tell Eve everything he knows...only to get murdered shortly after his decision. Now that's just harsh!
In Warrior Cats, when Beetlewhisker says that he's going to leave the Dark Forest because he didn't know that they wanted to destroy the clans, Brokenstar leaps on him and kills him.
Shel Silverstein wrote a poem about two generals who didn't want to fight, but go to the beach together. They discussed it, thought about what people would say, and killed each other.
Cyric in the Avatar trilogy in the Forgotten Realms world has a brief battle with himself about his betrayal of his friends.
Theon Greyjoy in A Song of Ice and Fire seriously considers joining the Night's Watch, partially because it would help him escape the hundreds of people who want his head, but also because it would give him a clean slate. Cue Ramsay Bolton and his men... Subverted in that Theon gets a second chance to begin towards redemption, but only after being crippled mentally and physically.
Theon Greyjoy:A black cloak can't be turned. I'd be as good as any man...
An interesting posthumous example happens in The Reader. After Hanna passes away having killed herself just before she was due to be released, it's revealed that she'd amassed a huge amount of money over the years and she leaves a letter asking that the protagonist do with it whatever he feels is appropriate. He goes to the last surviving victim of Hanna's crimes, asking that maybe the money should be given to a Jewish charity... and she says no, because that would posthumously redeem Hanna, something which she refuses to do. He gives the money to a charity to do with helping Jewish illiterate people become literate instead.
At the end of Warlord Of Mars, the xenophobic Phaidor, who had long thought herself a living goddess who was above such petty things as morality and death, repents and apologizes to John Carter and Dejah Thoris for being such a jealous, psychotic bitch. She then resolves to start a new life... by diving off into a very deep crevice. She hadn't realized that the fall would almost certainly kill her. (An alternative and perhaps more likely explanation is that she knew very well it would kill her and was going for Death Equals Redemption.)
When Londo learns the truth, he becomes The Atoner, but gets the door slammed on his face again and again. Eventually, he plans a Thanatos Gambit with the only people who still believe in him: Vir Cotto and G'Kar.
There was an early episode of Xena: Warrior Princess where Xena, after encountering some of her former warriors back from her days as a Blood Knight, convinces one of them that her commitment to reform is genuine. Later on in the episode, that guy sacrifices his life to save her. Fast forward to a different episode, where Xena is asked by Hades himself to go to the underworld, because evil has achieved a Karma Inversion: all the blessed people who used to play in the Elysium Fields have been sent to Tartarus, and all the evil folks condemned to Tartarus are partying in Elysium. When she gets there, she finds her dead friend, and realizes that if he's in Elysium now, he must have been in Tartarus before. He confirms it, saying something like "One sacrifice wasn't enough to make up for a lifetime of evil." In the end however, he's allowed to stay in Elysium for good after helping save Hades.
In a flashback to Xena's Blood Knight days, she and her lover Barius plan to wipe out the Centaur race. Barius has a crisis of conscience and sabotages their plans, for which he is forever hailed as a savior by the Centaurs, but Xena murders him when she finds out what he's done.
Boomer of Battlestar Galactica, who switched sides more time than this entry will try to document, finally got the door slammed on her as the series approached the end. After faking a Heel-Face Turn and returning Ellen to the fleet, she steals Hera and returns to the Cylons. However, Boomer and Hera bond and she feels immense regret for what she's done, so she takes Hera and returns her to Athena, her real mother. Athena, sick of Boomer's bullshit, shoots her. At least Boomer was savvy enough to realize returning Hera was going to be the last thing she does.
Jonathan of Buffy the Vampire Slayer was the least evil of the Nerd Trio, and was not only willing to go to jail to pay for his crimes, but drag Andrew along with him. He only decided to flee to Mexico when Dark Willow attacked. He returned in Season Seven with vaguely benevolent intentions, but was killed by Andrew (who was under the influence of the FirstEvil) before he could make contact with Buffy.
A literal Heel Face Door Slam happened to Spike. Originally a villain, he became a reluctant ally of the good guys in Season 4, and in Season 5, having fallen in love with Buffy, he tried acting more heroic in order to impress her. However, when Spike expresses his love by kidnapping Buffy and chaining her up in his crypt, she has Willow cast a spell that makes it impossible for Spike to enter Buffy's home. He's initially surprised, then emotionally devastated, when he runs into the invisible barrier surrounding her house and Buffy slams the door in his face.
Spike got another chance later, though - refusing to tell Glory what she wanted to know (that Buffy's sister was the MacGuffin Girl she was looking for) at risk of death earned him a measure of Buffy's forgiveness. Though he'd still cause trouble for her and her friends at times, Buffy notably never reverted to the same level of contempt she'd had for him beforehand.
Also, Faith. An ally of Buffy, she defected to the other side. Later, she interrupted a captive Willow, expecting Willow to start begging her and trying to convince her that it's not too late. Instead, Willow tells her that yes, it is too late, and that Faith won't have any more chances for redemption. Don't worry, she eventually got her Redemption in the Rain, thanks to Angel. Willow even takes back her sentiments and helps Faith along in being good later.
Faith's initial turn to the dark side came when she performed an (unwitting) Heel Face Door Slam on a minor character - the Deputy Mayor, more a 'snivelling lackey' than an actual Heel, tried to bring information on the Mayor's masterplan to her and Buffy, but he surprised them mid-fight and Faith staked him on instinct. The downward spiral this sends her on ends with her becoming the Mayor's Dragon.
Over on Angel, Lindsey gets one of these when he tries to leave Wolfram & Hart. Angel first doesn't trust him, then doesn't care, and when he finally does agree to help Lindsey bring down the company, he winds up leaving him behind to get caught. While somewhat understandable, this still probably contributed to Lindsey's decision to stay.
Yeah, and then when Lindsey tried to join the team in the series finale, Angel had Lorne kill him.
Which might have had some (troubling) justification. Lorne had seen Lindsey's nature and told Lindsey that he would never be part of the solution.
On The Wire, while the heel/face lines are often very blurry to begin with, basically any time a character involved in organized crime decides to become an informant, they inevitably die. The most prominent example is probably Bodie Broadus, who spends the first three seasons of the show steadily rising the ranks of the Barksdale drug empire, only for that empire to crumble at the end of the fourth season leaving him on his own as Marlo Stanfield's far more brutal, violent regime takes over. Bodie chafes at this and by the end of the season he's convinced to inform on Marlo and company. He dies before he can ever actually go through with it, albeit on his own terms.
In an episode of Desperate Housewives, Orson's Ex-Wife Alma, who, up until then had been colluding with Orson's Mother to kill Bree, ends up falling off a roof and killing herself while trying to warn Danielle.
Maury Parkman in season 3 of Heroes gets his neck snapped by Arthur Petrelli when he realizes Arthur's evilness and rebels to try to protect his son, Matt.
An interesting case in Power Rangers Lightspeed Rescue. Diabolico, after discovering how little his mistress Queen Bansheera cared for her minions, even destroying one of them in an attempt to get the Rangers, vowed to never serve her again. However, he was brainwashed and forced to fight the Rangers. Despite knowing that he wanted to change, the Rangers had no choice but to destroy him. However, in the finale, he gets another chance at Heel-Face Turn by returning as a spirit and helping the Rangers imprison Bansheera in the demon dimension forever. (It should be noted, however, that at no point did Diabolico indicate that he was actually turning good, he was just turning against the Queen. He himself put it that he'd come to hate the queen more than them.)
Frax's fate in Power Rangers Time Force was even worse. He realizes the error of his Face-Heel Turn ways... just as he's being dragged off to have his humanity and memories wiped by Ransik. That's two for Ransik, given that he's the one that made Frax what he had turned into in the first place.
In Kaizoku Sentai Gokaiger, Sally has trouble deciding whether to return to Basco after what he put her through. Basco shows no remorse when arguing with the Gokaigers, and Sally ultimately chooses the Gokaigers. Basco's reaction? Without thinking twice, no remorse, and actually laughing while at it, he activates a bomb he planted earlier on Sally in case this were to happen. Cue his line "To get something, you have to give something up."
Dollhouse has this happen to Bennett. She's made the choice to help the team at the LA Dollhouse, if only because Echo has promised to help her get all the revenge she wants on Caroline and she's in love with Topher. Then Saunders shoots Bennett in the head.
In Once Upon a Time, every time Regina gets even close to redeeming herself the good guys shoot her down brutally, target her for a crime she hasn't committed, decide she's irredeemable, or threaten to take Henryaway from her permanently. There's generally a good reason why, or what seems like a good reason at the time, but at this point it's almost understandable that Regina seems to have given up her attempts to reform.
Wakana Sonozaki from Kamen Rider Double had planned on getting away from her megalomaniac father's plans and move out of the city with her friend/ brother Philip. Her dad had other ideas. After her father died and she was subsequently used as an Apocalypse Maiden by her father's ex-financers, her mind was so shattered that she was put under observation in a mental hospital. She finally gets her redemption by sacrificing herself so that Philip/Raito can be brought Back from the Dead.
Even though it was mentioned several times that the chances of a Zodiarts finding redemption is slim, Kamen Rider Fourze manages to contradict that statement. So far, Perseus, Musca, Aquarius and Taurus (the last two are high-level Horoscopes, even) have become good by the end of their arc...however, only Musca was able to get out of it unscathed. Libra brainwashes Perseus to fight Fourze, Aquarius gets Laser-Guided Amnesia, and Taurus is sent to the Dark Nebula for trying to defy Virgo.
On LOST, we have Charles Widmore. When he shows up claiming to have redeemed himself in season six, we aren't quite sure if he's legit or not. When we finally realize that he is, he is promptly shot and killed by Ben, though that's more because he didn't want Widmore to have a chance to save his daughter when Ben didn't get a chance to save his.
In Legend of the Seeker TV series, just as it seems that Zedd has talked Denna into changing her ways and setting him free, she is shot in the back with an arrow by Cara and falls off a cliff.
Morgana in Merlin has two. At the end of the second season she is unknowingly the source of an enchantment that has put everyone in Camelot to sleep. The only way to break the enchantment is to kill the source - so Merlin poisons her. Throughout that season she had been teetering between good and evil but this act of betrayal acts as a catalyst. So when she returns a year later, her half sister Morgause has corrupted her completely. Then in a season 3 episode she discovers that she is Uther's daughter. When he refuses to acknowledge her, it's a pretty definitive door slam.
This happens to Cole in the fourth season of Charmed. He finds a wizard willing to take the Source's powers, thus freeing him from being possessed but Phoebe appears on the scene and kills the wizard before he can take the powers. Cole is killed in the next episode.
Then when he returns in Series 5, having broken out of the Demonic Wasteland, he's promptly declared evil by the sisters for his actions whilst being the Source, despite being possessed at the time. This rejection coupled by Phoebe's repeated threats to kill him for even the slighest infractions, causes him eventually to lose his mind and turn evil again. He's then killed... again.
Kira the Seer was a particularly tragic case as she was a demon who wanted to become human. Unfortunately, just before the ritual could be performed, she was destroyed by Zankou. Phoebe, the one she was closest to, took it the hardest.
The Walking Dead: During Rick and Shane's confrontation in the field, Rick tries to convince Shane into putting down their guns and going back to being friends again. Rick used this opportunity to get close enough to kill Shane with his knife.
In 24, if you're a bad guy and you decide to switch sides for whatever reason, you're odds of dying go way up. Farhad Hassan is shot in the back in the episode after he decides to help CTU. Dina Araz is given orders to execute Jack but refuses, which results in her getting executed. Josef Bazhaev is shot SECONDS after agreeing to turn himself in.
A particularly brutal example with terrorist mastermind Hamri al-Assad. He decides to forsake terrorism and work with the US government to capture his former lieutenant. Six hours later, he's killed in an assassination attempt on the president. Made by disgruntled members of the US government no less, and then he gets blamed for it. Made worse by the fact that Jack was forced to kill Curtis Manning to bring al-Assadin, which means that that happened for nothing.
"Apologize" by by One Republic (featuring Timbaland). The refrain says "I said it's too late to apologize, it's too late." Granted, it's probably less along the lines of "you can't have a chance to change your ways ever" and more like "this relationship is over regardless of whether you say you're sorry" or "if you want to have a relationship where you're not a traitor, it won't be with me."
The book of Revelation from The Bible points out that once a person has taken the Mark of the Beast and has worshipped the beast's image, they're screwed for eternity.
After betraying Jesus and regretting it, Judas tried to get the Sanhedrin to reverse the transaction, but they would have none of it. He was Driven to Suicide as a result.
In Dino Attack RPG, this was Pterisa's greatest fear. She knew that having a place in a post-war society meant that she had to make sure Dino Attack Team trusted her and knew that she was on their side; however, she feared that the reputation of Quasifigus Hybrids meant that, if they discovered her identity, she would forever lose her chance at gaining their trust. Once the Darkitect figured this out, he attempted to slam the door in her face by revealing her identity and reminding the present Dino Attack agents of the Quasifigus Hybrids' untrustworthy history. Fortunately, thanks to Andrew, Amanda, and Rex holding the door open, it didn't work.
Edmund, upon realizing he is mortally wounded, wants to do one good thing before he dies. It doesn't work.
In the opera Susannah, Olin Blitch, the traveling evangelical preacher, forces the protagonist (who is not on good terms with the rest of the town) to have sex with him. Overcome with remorse, he tries to convince the people that she's not a bad person. They don't listen. He begs Susannah to forgive him, she doesn't. Then her brother kills him.
In The Crucible, Mary Warren told the judges and everyone that she and the other girls were lying about being attacked by witches. Then she got scared and ended up accusing John Proctor of witchcraft instead.
Truth in Television: Mary Warren did this in real life, from what historians know. She admitted that she and the girls were lying, but then they accused her of witchcraft, saying she'd joined the witches so they'd stop hurting her. Mary confessed to save herself, and "confirmed" the girl's accusations as true.
Nuwangi in the original Utawarerumono visual novel, where after being defeated he vows to end the war... and is promptly killed by bandits for the reward they assume they'll receive. The anime just had him disappear.
If you defeat Darth Malak of Knights of the Old Republic he may have some regrets to share with you as he lies dying, "Still... still spouting the wisdom of the Jedi, I see. Maybe there is more truth in their code than I ever believed. I... I cannot help but wonder, Revan. What would have happened had our positions been reversed? What if fate had decreed I would be captured by the Jedi? Could I have returned to the light, as you did? If you had not led me down the dark path in the first place, what destiny would I have found?"
The player is given a chance to throw that back in his face, however: essentially saying something along the lines of, "I turned back to the Light, you could have as well. You chose your own fate."
The player can also pull this on Bastila if you are a real Jerk Ass.
You can also do this to Atris in the sequel.
In Neverwinter Nights, if you can turn Aribeth back, she comes back, turns herself in, and tells the leadership of Neverwinter everything she knows about the Luskan forces. Then they turn around and execute her anyway.
A really mean player who chooses to side with Kelemvor at the end of Neverwinter Nights 2: Mask of the Betrayer can do this to Arraman.
Bishop has the door slammed in his face as well. He gives you a piece of the Mask and warns you of enemies that are about to attack you, implying he's done at least some soul-searching. Unfortunately, he's absorbed into the Wall moments later.
After dueling Loghain in Dragon Age: Origins, you can offer him a chance to redeem himself, but generally the path is to decapitate him right there, if only because Alistar will leave the party in disgust at allowing the man who led nearly every Grey Warden of Ferelden to their deaths live. It's worth noting that Riordan, the person who suggests to let Loghain be of some use instead of executed, is a Grey Warden himself.
The end of the Best Served Cold quest in Dragon Age II. Thrask forms a secret mage-templar alliance to depose Meredith and get the Circle working the way it's supposed to. Then he has one of Hawke's loved ones kidnapped just in case they try to stop him. While he has no intention of actually hurting the hostage, his right-hand woman doesn't share his good intentions, and turns on him the second he stands in her way.
Special Agent Kato in Shadow Hearts Covenantgradually grows disgusted with the motives of his superiors, and shifts focus towards his own dream of having a family with the clone of his murdered love (it's complicated), and living peacefully. Then the girl is killed a second time, and Kato decides to destroy the world.
In Radiant Dawn, Sephiran is so utterly resigned to his fate that the only way to save him is if you start a New Game+.
Another canon example from Sacred Stones: Glen and Duessel, two of Vigarde's generals, both figure out that something rotten is going on in Grado. Duessel subverts it and successfully defects, but Glen is killed by Valter before he can switch sides.
Kuja of Final Fantasy IX redeems himself by teleporting the heroes out of the Hill of Despair and outside the Iifa Tree. It's probably because of this that Zidane decides to go back for him. If anything, at least Kuja didn't die alone.
Also Benezia and Fai Dan from the first game. All examples save the last have the same source. you can also choose to slam the door in the Rachni Queen's face in 3.
In Mass Effect 2, if you talk Miranda down from killing Niket, who, after hearing of Miranda's plan to, in his view, steal Oriana from their father, betrays Miranda to the Eclipse mercenaries, he sees the error of his ways, but is gunned down immediately afterward.
In Mortal Kombat, this happens to Cyrax, who gets a chapter in Story Mode showing his signs of turning good, ditching the Lin Kuei over the Cyber Initiative. Then he got captured and roboticized off-screen.
Elder Sub-Zero dies at Scorpion's hand and his soul descends into the Netherrealm to become Noob Saibot (as foreshadowed in Mythologies and revealed in Deception). Raiden tries to avert this in 9 by striking a deal with Scorpion (if Scorpion doesn't kill Sub-Zero, he'll ask the Elder Gods to restore Scorpion's family to life), but Quan Chi, being the asshole that he is creates false images of Sub-Zero killing Scorpion's wife and child, causing Scorpion to lash out and kill his nemesis in anger anyway. So, not only does the younger Sub-Zero get dragged into the picture alongside Smoke again, but his older brother returns to life as Noob Saibot again. This also comes with the unfortunate side-effect of Scorpion playing Quan Chi's lapdog, something that Raiden calls him out on (in the original timeline, Scorpion learned that Quan Chi, and not either of the Sub-Zero brothers killed his family, and chased Quan Chi through the Netherrealm after 4, beating the tar out of him).
Also, Sindel. True to Mortal Kombat 3, she's brought Back from the Dead as a part of Shao Kahn's Evil Plan, Brainwashed and Crazy. In the original timeline, she was able to break free of her mind control and was reunited with her daughter Kitana and their allies. Here, Shao Kahn empowers her with Shang Tsung's soul, allowing her to kill 90% of Raiden's chosen warriors (including Kitana). It takes a kamikazeHeroic Sacrifice on Nightwolf's part to kill her, and her and the fallen heroes' note Kitana, Nightwolf, Cyber Sub-Zero, Smoke, Jade, Jax, Stryker, and Kabal; Kung Lao had been killed by Shao Kahn previously, while Raiden and Liu Kang were away consulting the Elder Gods; only Sonya and Cage made it out alive souls descend into the Netherrealm, where they become Quan Chi's slaves. Sindel does make a Heel-Face Turn in her Arcade Ladder ending, but this is noncanonical in regards to the story. note Although it does provide a good What Could Have Been and the true nature of Sindel (which is good, not cackling evil), which gives a proof that the reboot doesn't destroy her whole characterization, fate just becomes much unkinder for her.
Grand Theft Auto: San Andreas has Officer Hernandez, the third, silent member of C.R.A.S.H (Well, not completely silent. The one time he speaks is on the phone with C.J. warning him that C.R.A.S.H is going to make sure he stays in Los Santos and works with them.) who acts as a Foil to the loudmouthed Tenpenny and Pulaski. Eventually however, he got fed up with Tenpenny and Pulaski's ways sometime offscreen as his final cutscene is of Tenpenny beating him with a shovel for being a snitch, and having C.J. dig his grave at gunpoint. Hernandez isn't dead though, and he gets back up to try and tackle Pulaski, only to get shot, finally dying.
The Introduction DVD expands on this. Hernandez speaks in it, telling a story about how he had to make a difficult decision between letting a man beat his crack addicted wife, or jailing the man and leaving the obviously unsuitable for children wife with their kids, establishing himself as a cop with morals. The other two cops scoff at that being a difficult decision, telling him that they're gonna be making difficult decisions on a regular basis. Later on, they force him to personally shoot a cop who had evidence proving that Tenpenny and Pulaski were crooked, so he can "be a fucking man." It makes Hernandez's death all the more tragic, as he was pretty much one of the handful of good cops in the whole series.
Gaspard in Dark Cloud 2 realizes that he's lost his way after a Not So Different moment with the hero and decides to quit serving the Big Bad and back out of the conflict entirely, expressing a wish to see the end of the heroes' journey and maybe end his own with them. Unfortunately for him, the Big Bad pulls a Villain Override out of spite and the heroes are reluctantly forced to kill him.
Specialist Cross in Prototype helps Alex Mercer defeat Blackwatch in the later half of the game. He is killed by the Supreme Hunter before he gets the chance reveals his true identity to Alex.
The Elder Scrolls IV: Oblivion: The Meherun's Razor DLC involves a mad wizard seeking an artifact weapon as part of his bid to invade Cyrodil. Deep inside his fortress, you encounter Khajiit workers bussed in by the wizard to help work his mines and outfit his army. Of course, they attack you on sight just like everyone else, despite the fact that they're not wearing armor and most of them have nothing but hoes and rakes to attack you with. While poking around, you can find a letter home by one of the workers saying that they hate Cyrodil and the work sucks, but the money's too good. It also mentions that they're planning rebelling on at very least against the head smith in the near future. Of course, that worker is probably dead now...
Much like the Penny Arcade example below, there's a lot of talk about missing the recipient of the letter and sending them needed money.
In The Elder Scrolls V: Skyrim, the Blades want you to do this to Paarthunax, the dragon leader of the Graybeards, for his past crimes... nevermind the fact that the only reason the atrocities were thwarted was because of his eventual notion that these mortals are worth something, and teaching them how to overcome Alduin.
The Bonfire of the Vanities DLC for Assassins Creed II does this several times in the same mission. In order for the people of Florence to rise up against the insane monk Savonarola, Ezio has to assassinate each of his lieutenants that he had corrupted with the Apple of Eden. By mortally injuring them, the men are freed of the Apple's influence, and several of them express remorse and horror for their actions. But by then it is too late for them to right their wrongs, and they die in misery and shame.
Notably, one of them admits that he wasn't under the Apple's influence and followed Savonarola because he happens to agree with him. He laughs at Ezio for thinking that the Apple is the only reason someone may share another person's ideas.
Cetsa gets this late in The Way: rescued from death and touched by both the hero's utter devotion to finding Serena and his utter disdain for Cetsa herself, she decides to start over anew and give up her life of crime. The first person she encounters after making this decision is a dying Slade who she doesn't recognize. Grappling with his own failures, he kills her so he can die knowing he did something right. Then everything starts getting weird.
Lor'themar Theron in World of Warcraft became increasingly angry with how the Blood Elves were treated by both the Forsaken and Warchief Garrosh. During the Landfall storyline Variann Wrynn made secret overtures to the Blood Elves in hopes of regaining their lost ally (the elves had left the Alliance after the Second War), and Lor'themar was willing to listen... until Jaina purged Dalaran of all Sunreavers and aligned the neutral Kirin Tor firmly with the Alliance after hearing of the Horde using Dalaran's portal network to steal the Divine Bell and blaming the Blood Elf Sunreavers.
Perhaps this goes on to be something of a subversion after the fact, as the Kirin Tor's actions may have soured the Blood Elves from rejoining the alliance, it hasn't stopped them from turning on Garrosh for his abuse of them. Further, his problems with Jaina certainly didn't stop him from finding a tenuous truce with her on Thunder Isle.
Nimdok's scenario in I Have No Mouth and I Must Scream entails realizing the atrocities he committed during World War 2, and doing everything he can to make amends in AM's recreation of the concentration camp he performed his research in. Neverthelress, in an ideal playthrough of his scenario, while his change of heart is recognized by the Jews he frees, they still order the Nazi's Golem to kill Nimdok once he relinquishes control of it over to them, as his crimes merited no less as punishment.
Miko Miyazaki plays this trope straight. Redemption in this case would have meant admitting she was wrong, and she dies before she gets the chance.
That said, it's implied that she isn'tgoing straight to hell, because she does genuinely care for innocent lives.
The prequel book "Start of Darkness" reveals that Redcloak falls into the non-lethal type. For a brief moment he considers setting his Evil Plan aside in favor of an ordinary life, but then Big Bad Xykon shows up and slams the door in the most tragic way possible.
Half-orc ninja Therkla gets this in a big way. She falls madly in love with Elan, but he's not willing to leave Haley for her, and nobody else on the Good side is willing to cut her any slack. She dies tragically.
In her case, it's arguable whether Therkla really wanted to redeem herself. She wanted to be with Elan, certainly, and was willing to switch sides to enable that- but her pre-death speech makes it pretty clear that he was the only reason she was willing to do so. When he refused to cheat on Haley with her, she basically gave up.
Homestuck: Vriska Serket began to regret the endless wave of pain and misery caused by her actions after killing a friend, and began opening up to John and expressing that all she wanted was to leave it all behind and and take a few cues from mankind on how to live, but before all that wanted to confront JackNoir to try and save her friends. Then Terezi killed her to prevent the doomed future that would've come from her attempt to fight Jack.
The plot of Darwin Carmichael Is Going to Hell can be summarized as "futilely banging on the door." Darwin only made one mistake, but the laws of the universe aren't letting him make up for it, since the incredibly bad luck he's been cursed with applies even when he's trying to help other people.
Seymour attempts to do this to Fuschia in the "Bad Behaviour" strips of Sinfest. If not for Criminy, it would have worked. Is a major reason why he has a Hatedom.
Played for laughs in Servants Of The Imperium#34. A pair of dark eldar decide they don't like being evil torture-loving psychopaths and that they want to leave. Next panel the heroes machine-gun them.
Brianna: I don't know what they were talking about, but I bet it was evil.
Link was going down this path in There Will Be Brawl, realizing that he and Zelda were going too far to get their hands on the Mushroom Kingdom throne. Then Zelda literally stabs him in the back with the Master Sword and leaves him for dead. He survives this however, and gets an actual Redemption Equals Death.
Zelda follows him down this road in the finale, in which she attempts to sacrifice herself in place of the "not yet dead" Link. Unfortunately, Ganondorf's attack is so powerful, it takes them both out simultaneously.
In the Global Guardians PBEM Universe, Armageddon Girl slammed the door on her own Heel-Face Turn. Ultra-Man managed to talk her down from destroying a city. The hero speaks to her with heartfelt concern about her former career as a teen sidekick to a superhero, and how no won could blame her for going a little nuts after her family was killed, and that she could get help if she only wanted it. She thought about it for a while, then tearfully smashed Ultra-Man through a building, crying, "No, it's too late for that."
In Avocado's defense, that was the most competent he's ever been at anything.
Survival of the Fittest has a definite example in v4, with minor player Remy Kim. After leaving behind one person to die in a dangerzone after stabbing him and then also stabbing a battered girl, he runs into Sarah Tan, his friend from school, and has a Heel Realization. He vows to protect her, and it looks like she might be a Morality Pet - type figure... but not only is she angry at him for his actions, but he promptly gets headshottedmidsentence by Ericka Bradley. Damn.
Lyn Burbank spends her early part of the game trying to play the game after killing her cousin, and even after she joins a group to repay being rescued from a nervous breakdown, it eventually falls apart, ending in her MercyKilling one of the members and pulling a gun on the other one before leaving him behind, going on to return to playing the game. After she gets caught and brutally tortured by J.R. Rizzolo and left to die in a burning hanger, however, she gets rescued by her former party member, leading to her finally breaking down, tearfully admitting that she's sick and tired of trying to play the game and that she just wants to go home. Unfortunately, she ends up dying of blood loss shortly after.
Non-death variant; Reiko Ishida, after her murder of Carol Burke among other things, becomes more hesitant to play the game, and once she finds her girlfriend Sarah Xu, she's obviously happy. However when the escape boats arrive she is denied going with Sarah, on the basis that she was playing the game to begin with. It ends with Sarah going on the escape boat (though reluctantly) and Reiko staying.
In Program V2, Damien Stone has a breakdown after killing a third classmate, realizing that what he's doing is wrong, and resolves to change his ways. These thoughts are interrupted by Dylan Walker shooting him in the back of the head.
In the first season of Video Game Championship Wrestling, Little Mac quickly became the promotion's top heel after winning Money In The Bank under dubious circumstances, and very quickly aligned himself with Corrupt Corporate Executive and VGCW owner Baz McMahon. This prompted a long feud with Zangief, who felt that he should have been the winner. The feud ended on the season finale, when Little Mac won an Iron Man Inferno match over Zangeif 8-4. At the end of the match, Baz threw a chair into the ring and ordered Little Mac to finish Zangief off, but Mac had a change of heart, threw the chair back at Baz, shook Zangief's hand, and ended their feud on a high note in a showing of mutual respect. It looked like Mac had a career as a babyface ahead of him until he was the victim of a hit and run on his way out of the arena.
What makes this worse is that afterwards, Phoenix Wright headed up an investigation to determine who was responsible, but at the end of the second season, his investigation's focus turned to finding the identity of Mr. L, while Mac himself became an afterthought. No one knew who was responsible for taking out Mac, and Mac himself has not been seen until Season 4 when he returned from his long recovery. The season also revealed that it was Phoenix who ran over Mac thanks to Time Travel.
In Avatar The Last Airbender, Zuko, who has been trying to capture the protagonists for three seasons and half-ignoring their insistence that he could be their friend for just as long, finally sees the error of his ways and, guilt-ridden and earnest, tries to join the Aang Gang so he can play a vital role in teaching Aang/saving the world. The protagonists unanimously decide that he's used up his chances and send him away. Then he accidentally injures the only protagonist still in doubt about him. Kid's got bad karma. All is eventually well though!
More accurately, he chased them for one season, had two relatively neutral encounters, betrayed an almost-not-really-there possibility of alliance, was absent from their lives for about half a season, and then turned up wanting to be their friend. Special in that he wasn't able to do this until after experiencing the shock of doing the thing that makes them unable and unwilling to trust him. And then the feet-burning thing.
Taken to ridiculous extremes in The Fairly OddParentsTV movieChannel Chasers. At the end of the movie, Timmy's Sadist Teacher Mr. Crocker was seen coming out of therapy, cured of his obsession with fairies and decides to turn his life around. He even discovered cold fusion! Then, even though he had nothing to do with the plot (in fact his very appearance was probably just to show he's a Cosmic Plaything), Timmy's wish that nobody remembers anything that happened from when the film began affected him, returning him to his fairy-obsessed self who tosses his cold fusion generator away because he doesn't know what it is anymore.
Done again in the episode "Crocker Shocker", where Mr. Crocker is put under hypnosis by a desperate therapist to erase his belief in fairies. Among the changes he undergoes, he loses his hunchback look and his ears move from his neck to his head. Unfortunately, it turns out that Fairy Magic is powered by Crocker's fairy believing spazz attacks, so Timmy had to get him to change back to his old self. You know the universe is unfair when one person's sanity has to be sacrificed.
In ReBoot, the treacherous binome Cyrus was on the point of a Heel-Face Turn at the end of "Firewall" after the protagonists guided him through saving the day from the Game Cube Of The Week. He was in the middle of being lauded as a hero when the Big Bad showed up and had his goons drag him onto their side of the eponymous impenetrable wall; a couple episodes later he was shown about to be executed. One of the goons set him free, but he still didn't show up again until a cameo at the end of the season.
One episode of Samurai Jack features the robotic assassin X9, who was one of several murderours robots created by Aku, but was the only one given emotions and feelings (As he explains it, the scientist who built him "was funny like that"). After years in the service of Aku all the other robots of his series have been destroyed, but he has survived because of his emotions. However, when he meets Lulu he finally hangs up his assassin hat for good, determined to settle down and spend his time playing music. Unfortunately, when Jack arrives, Aku becomes desperate and decides that he has to pull his greatest assassin out of retirement by holding Lulu hostage. Jack knows nothing of this, and when X9 launches his attack he is quickly destroyed, his final words are asking Jack to finish caring for his now abandoned charge.
In Transformers Prime, Starscream gets so sick of the being the Decepticon's Chew Toy that he tries to defect to the Autobots. Note that Starscream is not in any way repentant, he just figures he has a better chance on their side. Once it's revealed that Starscream killed Cliffjumper, Arcee goes ballistic and tries to avenge her dead partner. Starscream is driven off, and is convinced that he should become a neutral party in the war.
Simon Squealer in Geronimo Stilton experiences this thanks to a combination of Negative Continuity and Status Quo Is God. He pretends to be a superhero called the Gator Samaritan in one episode, and the heroic acts he performs are staged to sell issues of The Daily Rat. But after he rescues Thea Stilton from drowning, he realizes Good Feels Good and starts disobeying Sally and performing genuinely heroic acts independently of her paper-selling scam. Although Thea is grateful enough to him that she lets him go after he's outed as a phony, and he is clearly feeling morally conflicted over his continued involvement with Sally, the episode closes with him still under her thumb (complete with a humiliating joke at his expense), and, the next time he appears, he's back to his old sniveling, weaselly self.
Non-lethal example: This U.S. Supreme Court case allows slamming the heel face door on "sexually dangerous" criminals on a federal level by allowing them to be kept imprisoned after their sentence is up. On the other hand, the Supreme Court found in this case that slamming the heel face door on teenagers who haven't killed anyone by giving them a sentence of life imprisonment without parole is "cruel and unusual punishment" that is constitutionally prohibited.
Sabino Arana Goiri (1865-1903) was a Spanish writer, philosopher, and political activist of Basque descent. It was he who founded the Partido Nacionalista Vasco (PNV), which was the first political party to strive toward an independent nation-state for the Basque people. He also harbored an obsessive loathing for any Iberian peoples who were not Basque, condemning them in his nationalistic tracts and arguing against intermarrying with them to such a degree that his attitude bordered on racism. (It should be pointed out that this was the 1890s, when racism was not only socially acceptable but also considered rational and scientific, not to mention ubiquitous; and anyway, most of Arana Goiri's opponents shared similar attitudes.) Arana Goiri eventually began to moderate his extremist views, deemphasizing race and stating that home rule for Basques within the Spanish nation-state would be an acceptable alternative. Unfortunately, he died before he could convince most of his followers to similarly adjust their attitudes, and the PNV (or at least a militant wing of it) remained radicalized down to the present day. It is largely for this reason that Basques are often thought of (at least by other Spaniards) as terrorist bombers, rather than the peaceful, churchgoing farm folk they always have been and still are. When a bombing occurred in Madrid in 2004, Prime Minister Jose Maria Aznar even hinted that the plot might have been orchestrated by ETA (a major Basque terrorist organization, currently at an extremely low ebb) rather than by the true suspect, Al-Qaeda. Many Spaniards, including non-Basques, conceded that that was wrong, and it's widely credited for Anzar's party being defeated in the national elections 3 days later.
Albert Speer (according to his own words, anyway) regretted being a part of the Nazi party, only a little too late to really do anything about it. He was sentenced to 20 years. (The tribunal was understandably skeptical of his regret.)