Love doesn't lead to the dark side. Passion can lead to rage and fear, and can be controlled, but passion is not the same thing as love. Controlling your passions while being in love, that's what they should teach you to beware, but love itself will save you, not condemn you.
Subverted in YuYu Hakusho: villain Sensui still tries to do nasty evil things despite being in love with Itsuki. Yusuke remains a jerk through most of the series even though he's in love with Keiko... though you could argue that Yusuke might be on the level of the villains without Keiko's mix of nagging and affection.
Played straight platonically with Kurama and his human mother, Shiori Minamino. He fused with her unborn baby and then planned to abandon her once his injuries were fully healed, but seeing her care for him is what made him one of the good guys.
Yusuke's ancestor, Lord Raizen. A member of a human-eating species of demon, he fell in love with, and then had sex with, a female doctor in medieval Japan, who had the guts to stand up to him when he tried to eat her (whose lineage later led to Yusuke). When we meet him, he's dying from starvation from refusal to eat human flesh in honor of the lady doctor, who died when she gave birth to their kid aka Yusuke's ancestor. Yusuke, being his most powerful descendant, becomes his successor after Raizen's death.
"It seems... we are not gonna have our Chocolate Parfait."
This repeated itself in Sailor Moon R with Saphir and Petz, though Petz was already a redeemed villainess. And Petz herself was redeemed because of the love of her younger sisters Berthier, Koan and Calaveras. In fact, except for Koan, all the Ayakashi sisters had their Heel Face Turns as their siblings tell them that they're not alone and they can have a peaceful life as humans.
Subverted in the case of Kunzite and Zoisite. Yes, they were in love, but they never even considered a Heel-Face Turn. In fact, given that Zoisite outright stated that everything he did was for Kunzite, and Kunzite later swore that he would get revenge for Zoisite's death, their love motivated many of the things they did.
The gentlemanly nihilist Celestin in Ah! My Goddess was redeemed, not by anyone loving him directly, but by feeling the love between Keiichi and Belldandy (he'd used Grand Theft Me on Keiichi, at the time). Knowing such a thing could exist, he regained faith in existence and ceased his attack on reality.
Yu-Gi-Oh!: The love between Seto and Kisara saved Seto from accepting the lure of The Dark Side and later from being possessed by his father.
Subverted in season two of Code Geass, when Rolo, Lelouch's fake brother/would-be assassin, does his Heel-Face Turn. Lelouch saves Rolo's life, then tells Rolo that even though the memories of their relationship were fake the feelings were still real, and Rolo 'isn't suited to death and violence.' Since Rolo had been showing signs of attachment to Lelouch, and his life before posing as Lelouch's brother had been one assassination mission after another, this of course causes him to switch sides. Lelouch's next internal monologue, however, reveals that he had staged the whole thing in order to convert Rolo, and that he fully intends to 'use him up and throw him away like a dirty rag', as a kind of revenge for daring to replace Nunnally.
The cruelest part? The trope was played straight with the other party. Rolo goes from a Creepy Child assassin to an adoring little brother. In episode 19, when Lelouch was betrayed and abandoned by everyone, Rolo was the only one who stayed with Lelouch, even after Lelouch admitted he was just using Rolo. Then, Rolodied.
Plays a major role in all parts of Robotech / Macross series: The Zentraedi, Meltrandi, Robotech Masters and even Invid make a Heel-Face Turn upon discovering love.
Used in Fist of the North Star repeatedly, often crossed over with Redemption Equals Death. Especially obvious in the case of Raoh. Just as Yuria admits she's about to die, Raoh realizes that he loves her. However, he really, really, wants to fight Kenshiro. More than just about anything else, including conquering the whole world. So he hits a few pressure points, ensuring that Yuria will live a few years longer, and heads off to his final hopless-battle knowing that once his little brother kills him, Yuria will be loved and taken care of.
In Cowboy Bebop, the implication is that once upon a time Spike and his Evil Counterpart Vicious were the same until the love between Spike and Julia turned Spike good.
In Fruits Basket Kagura confesses that she pursued Kyo because she thought that loving him would cancel out some cruel and selfish behavior toward him, but now she realizes that it was as selfish as the original actions.
Both Kyoko and Kyo became happier, nicer people after Katsuyo and Tohru respectively entered their lives, as those were the first people to treat them so well.
In the Fatal Fury first movie, young Lily Maguire was a pawn in Geese Howard's plan to murder his old rival Jeff. She grew up into a gorgeous and flirty Femme Fatale, but could never forgive herself for her role in Jeff's death, until she met Jeff's eldest son Terry and gradually recovered her hope in life. But since this is Terry Bogard we're talking about...
Digimon Adventure - Yamato usually only gets a grip and starts acting like part of a team after someone else (specifically Taichi, but there are other examples - Yamato was a bit slow on the uptake) nearly gets themselves killed trying to help him and his Crest activates. He has a few snap backs until finally getting the hang of the whole thing, at which point there's the whole holding hands and being shot with arrows of concentrated love thing...
Yu-Gi-Oh! GX Once Juudai remembers his vow to always love Yubel, it's that love that leads him to choose Super Fusion to add to her hand, giving him the chance to fuse their souls together, purging her of the Light of Ruin, redeeming his mistakes and hers as well.
Boa Hancock of One Piece starts off as an absolute dog-kicking bitch to everyone except for her two younger sisters. However, after falling in love with Luffy, she begins to turn around, and eventually becomes an overall nice person (to her allies, anyway). Not to say that Luffy returns her feelings, but still.
Vegeta sacrificed himself in Dragon Ball Z in order to save his loved ones in a Redemption Equals Death way. Goku later used his feelings for Bulma as a way to get him to use the fusion earring.
While it never really convinced them to become permanent good guys, Pokémon's Team Rocket care enough for their Pokémon (and each other) that they go out of their way to help them in times of trouble. (see "A Poached Ego" and "Crossing Paths" for some excellent examples, although you should also bring a tissue)
In Fairy Tail, as the Tenrou Island Arc begins to wind down, Ultear and Meredy are leaving the island via boat, where Ultear confirms that she was the one who attacked Meredy's village. She then stabs and throws herself overboard, her My God, What Have I Done? moment overtaking her will to live. However, Meredy uses her magic to learn exactly how Ultear feels, and saves her from death while saying they can forget about Grimoire Heart and live on together, coupled with a declaration of love. Many Awws ensued.
In Kamisama KissJirou's affection for Nanami mellow him out quite a bit. Looks like Nanami is responsible for it happening to Tomoe as well.
A running theme of Apollo's Song. The futuristic arc has the queen of the artificial humans become kinder and more gentle after she falls in love with Shogo. Similarly, the storyline where Shogo and Hiromi are stranded together on an island, their blossoming love causes them to be much less selfish. The main storyline, meanwhile, deals entirely with Shogo's psychotic behavior being healed through learning to accept love, which turns out to happen when the Hiromi of that period is actually a student of Shogo's psychiatrist, sent to keep an eye on Shogo and teach him love after he escapes the institute he was kept at.
Subverted at several points in Anatolia Story, when the protagonists discover the parts love played in the bitterness and corruption of the two villains, Nakia and Nefertiti. Even though both were shown as being the victims of bad circumstances (forced into arranged marriages for the good of their home countries) and were shown at their nicest in flashbacks with their lovers, it's made clear that they've grown to too far gone to even consider redemption. This is shown through how Nakia is willing to sacrifice Urhi, should it protect herself, and how Nefertit completely forgot the significance of a momento she gaveher brother.
A Certain Magical Index: Whether or not Accelerator even can be redeemed for the things he's done is a point of contention both in and out of universe, but it's clear that Last Order's love for him (and his for her, though he rarely admits it) is one of the only reasons he's improving at all. Similar things begin happening with Shizuri Mugino, although Shiage doesn't return her feelings and she is aware of this. Furthermore, Accelerator's strange friendship with Misaka Worst pulled her out of a suicidal rampage and seems to be bringing out some slight improvements in her behavior.
In Aquaman, the post-Brightest Day origins of Mera, Aquaman's wife. Once the princess of Xebel, an ancient penal colony of outcasts and rebels from Atlantis, she was born, bred an raised as a tool to find, infiltrate and destroy Atlantis from within, starting with the then current king. While feigning closeness to Orin, however, she started to get real feelings for him, eventually defecting from Xebel and becoming a powerful heroine on her own.
In Invincible, the love of his family ultimately changes Omni-man for the better, though he spends a long time in denial. Other Viltrumites start falling for this too when they become emotionally attached to the humans with whom they are siring a new generation of Viltrumite hybrids.
In The Sun Soul, Misty's fight against Team Rocket goes from a quest for revenge against the former teammates that tried to kill her to an outright Heel-Face Turn because of love for/from Ash.
The MLP fanfiction series from 'Masterof Nintendo' seems to like this trope. Particularly where the OC Rick is concerned. At first, he was FAR from willing to change his insane and chaotic ways, but after a whole season living with Twilight Sparkle, he began to warm up to the place. It's when Discord corrupts Twi does Rick make a heel face turn and decides to use his powers for the greater good. Later, a top ranking demon falls for Pinkie, and rebels against the devil just to be with her.
Fluttercruel in the Pony POV Series, of the Parental Love variety. She is technically Fluttershy's and Discord's daughter, or at least considered such by herself and Fluttershy, despite that they're Sharing a Body. At first she behaved like Discorded!Fluttershy, but Fluttershy's maternal love ultimately turns her into a much better pony.
Subverted with Discord. Neither Celestia's romantic love for him nor Shady's maternal love were capable of redeeming him.
Nictis, the Changeling Infiltrator in Without A Hive. At first, Spark Wheel is just a convenient source of affection for Nictis' cover identity of "Meadow Song" to harvest. As the years pass, Nictis increasingly ''becomes'' Meadow Song, to the point of falling in love with Spark Wheel. Meadow Song's friendship and love for Spark Wheel causes Nictis to adopt Pony values, becoming by Pony standards basically "good" (and by Changeling standards essentially an insane traitor).
The author of the Facing The Future Series seemed to feel that the reason Desiree went bad because she was spurned. She eventually finds redeeeming love in Sidney Poindexter.
In Mega Mandefenderof The Human Race, ProtoMan is forced to kill Mega Man via the Mad Grinder, and seems helpless to do so due to Wily's override command. But memories of his brother, Nomad, and Mega Man never doubting that he loves ProtoMan enable him to turn his back on Wily, saving his brother's life and the world.
Film — Animation
Beast from Disney's Beauty and the Beast is cursed to look horrible by an enchantress. He doesn't get the respect of Belle until he starts acting kinder to her and the others, and ultimately their love is what breaks the curse on him.
The title character in Megamind made a Heel-Face Turn after falling in love with his former rival's love interest, Roxanne. It was because he made the turn that she fell for him.
In Tangled, Flynn's attraction to Rapunzel leads him to abandon his Greed, try to make things right with his old partners he betrayed, and face imprisonment and execution because of his desperate fears of what Gothel is doing to Rapunzel. And finally to die rather than let Gothel get Rapunzel's promise to stay prisoner if she can save him. Fortunately, he gets better.
In a slightly roundabout way; it's pretty much a given that Will's love for Elizabeth is what leads him to accept his new duties of ferrying the souls of the dead on the Flying Dutchman. This frees the crew from the corruption that Jones had caused with his actions and they also seem to lose their mindless bloodlust.
Angelica attempts to do this for Blackbeard. But he's a bad man.
Darth Vader, after a tense scene where he balances the death of his only son against his long-held dreams of power, makes his choice, abandons everything he has worked for, and sends the Emperor hurling down the Death Star reactor shaft. He then dies, redeemed. Ironically, love both redeemed and corrupted him, as he joined The Dark Side because of his love for his wife, whom he had prophetic dreams of death about. Too bad about the Prophecy Twist.
In the classic Beatles animated film, Yellow Submarine, the group is able to free Pepperland from the Blue Meanies. However, they immediately offer to forgive the Meanies if they join together in peace. True to the spirit of the times, the Meanies sincerely accept the offer and everyone celebrates with the music of the band.
In Unforgiven, William Munny (Clint Eastwood), known in his youth as an infamous gunfighter, murderer and bandit, has since retired having forsworn his criminal ways through the influence of his late wife.
In Salt, thanks to her husband's genuine love, Evelyn Salt had no intention of carrying out Orlov's orders. In fact, all her actions, including faking the Russian Prime Minister's death, was to save her husband.
In the original, silent The Thief Of Bagdad 1924, the thief magically gets into the palace and falls in love with the princess, leaving without taking more than a token. Then he disguises himself as a prince and tries to win her, and gives that up because it's lying. He is in despair until receiving the advice to become a prince to win her. He then sets out to win her properly.
In the James Bond movie Moonraker, the giant mook Jaws is redeemed after falling in love: the realization that Drax's plan to exterminate anyone not fitting the image of his "master race" would include tiny, glasses-wearing Dolly is what prompts him into his Heel-Face Turn.
Played with in Looper. Old Joe thinks this is what happened to him. While this is true to a certain extent, he's nowhere near as good as he'd like to think: love didn't redeem him so much as it turned him from an amoral thug into a Well-Intentioned Extremist.
A milder example in the same book occurs when Narcissa Malfoy lies to Voldemort about Harry's death, thus protecting him, as her only remaining motivation is her love for Draco.
This is the defining trope of Harry Potter. If a character truly loves another, then they become at least somewhat sympathetic. Inversely, all of the characters who don't love are pure evil. Well, not ENTIRELY. Bellatrix Lestrange, for instance, has a very powerful obsessive love for her master, Lord Voldemort, and is still quite evil, without a sympathetic moment to her name.
In Piers Anthony's Apprentice Adept series, the evil Tania falls in love with the protagonist Bane and has a Heel-Face Turn (and a serious personality change), though she ends marrying someone else as Bane is already married.
This trope is a biggie in the 18th-century novel Pamela by Samuel Richardson, a book that intended to be about proper conduct. A young waiting-maid's mistress dies, and so the household is given over to her son, Mr. B, who quickly sets his sights on Pamela's "virtue." She manages to refuse his advances despite his hostile behavior and attempts to full-out rape her, and eventually decides to quit and return home. Only the carriage meant to take her home kidnaps her to one of Mr. B's other estates, where she is held prisoner with her virtue as the ransom. She endures more of his advances, until finally Mr. B reads the letters she's been writing detailing her imprisonment and just then realizes what a terrible human being he's been. He lets her go, but Pamela realizes she's in love and accepts his marriage proposal. And suddenly Mr. B has been supporting an illegitimate child all this time, so he's really a good guy, honest!
In White Fang, without actually redeeming himself as he retains his wolf instincs, White Fang becomes much less wild and violent after being adopted by Scott, the only master he grows to love.
In the Sword of Truth series, both Denna and Nicci are redeemed by their love for Richard Rahl. Neither of them actually gets to be with him, as he was already with Kahlan, but Denna died shortly afterwards and Nicci clearly stated that I Want My Beloved to Be Happy.
Many fans of Tales of the Frog Princess by E. D. Baker theorize that this happened to Garrid. When you think about it, it actually makes sense. Pre-Li'l: Cold, cunning, a liar, and something of a Jerkass. Post-Li'l: Funny, relaxed, Friendly Neighborhood Vampire, very helpful ally to have around. Depending on your view, he could also have qualified as a Jerk with a Heart of Gold all along— it just took Garrid falling for Li'l for it to show.
In Death: Eve and Roarke certainly experience this from their relationship.
Played With on an ongoing basis in The Dresden Files - love can save a White Court Vampire before their first feeding, but once their demon has asserted itself, love is merely dangerous. Harry tries to invoke this for Molly Carpenter, his apprentice. He tries to avert the romantic aspect of that as much as possible (for obvious, squicky reasons), but seems to have failed as of Ghost Stories.
Turns out that it was true for Charity Carpenter as well, redeemed by her love for her husband.
Averted for Shadowman, Susan Rodriguez, and the Denarians.
Turns out that the Knights of the Cross exist to fulfill this trope, turning the hosts of the Denarians against their Fallen Angel. One of their swords is even named "The Sword of Love". Yeeesh. Don't think that they wont work with the occasional wizard who knows how to deliver the alternative for great justice.
The man that became Kthonia's husband was a Death Seeker and therefore Too Kinky to Torture. Eventually she fell for him and mellowed out.
Crime and Punishment: Raskolnikov is on his way to redemption at the end of the novel, thanks to his relationship with Sofia.
One of the most prominent themes in The Bible, with the Great Commandment in the New Testament being as love for God and love for your neighbor meaning ultimate redemption.
In The Phantom of Manhattan (a sequel to The Phantom of the Opera), Erik, the titular Phantom, realizes that his love for his son Pierre is a greater one than what he has for Christine, and when it is reciprocated he makes a full Heel Face Turn.
In Simon R. Green's book Hex In The City, a man called Sinner (who has been rejected by both Heaven and Hell, and as such must walk the earth eternally) and his succubus soulmate Pretty Poison are being attacked, and Sinner shields her with his body. As his body is being chipped away, piece by piece, by the constant attacks, Pretty Poison comprehends the notions of self-sacrifice and love, and instead moves to shield him with her own body. Cue a pillar of light that transforms her to her pre-Fall angelic state,and both are forgiven and raised to Heaven."Come with me, to Paradise," said the angel to the man called Sinner. "For you have been found worthy, as have I."
The Fifth Wave by Rick Yancey uses this trope when dealing with Evan Walker, who is a Silencer, who basically roams the countryside shooting people until he falls in love with Cassie.
This happens to Dime Novel villain Zanoni the Woman Wizard. Arch-Enemy Dr Quartz once kidnapped Nick Carter and brainwashed him into thinking that he was an invalid married to Zanoni. Nick's kind treatment of Zanoni made her fall in love with him and reform.
In a Real Life example from Survivor, Jonny Fairplay was recently invited back onto the show for its latest season, Fans vs. Favorites. Fans who remember Fairplay from the season he previously appeared on, Pearl Islands, remember him as a deplorable bastard who went so far as to lie about his grandmother's death to win an immunity challenge. During Fans vs. Favorites, however, he was noticeably subdued, mentioning his girlfriend and his unborn child several times, and then getting himself voted off the island so that he could return and be with them.
In Battlestar Galactica, Athena was supposed to seduce Helo into impregnating her so her race could get their hands on a human/cylon hybrid. However, she switches allegiances when she realizes that she had fallen in love with him. Caprica-Six starts a movement to change the Cylon plan because she's in love with Dr. Baltar.
In The Plan, we see that the titular plan of the Cylons unraveled because so many of them fell in love with humans and switched sides.
In "The Armageddon Factor", the Season 16 finale of Doctor Who, only Romana is able to stop the Doctor from doing a Face-Heel Turn after gaining the MacGuffinCosmic Keystone. The Shadow's reaction is epic. (Don't worry if you have no idea who or what the Shadow is. He's there to be defeated by Romana's love. That's all you need to know.)
This trope is supposedly what caused the Ninth Doctor to not be totally depressed/suicidal/batshit insane after the Time War because he met Rose.
It is because of how strong Amy and Rory love each other that Rory is able to come back to life with his soul and overcomes his impulses to kill. A bit too late for Amy, but she too gets better.
It runs in the family. River Song is redeemed from being a purely heartless and brainwashed killer by love.
Damon Salvatorebecomes less of a villain around the time he begins to fall in love with Elena.
In Kamen Rider Double, Ryu Terui loses his craze for vengeance partly because of his love for Akiko.
Kamen Rider Ryuki's Ren/Knight claims that his fiancée is the reason he gave up his previous life of delinquency and petty crime. He ultimately enters the Rider War to save her.
In Kamen Rider Decade, Natsumi's influence over Tsukasa is a major reason why he ultimately stops being the Destroyer of Worlds.
In Kamen Rider OOO Ankh is ultimately redeemed through his friendships with Eiji (who appears to love him in a True Companions way) and Hina (who has grown to love him as much as her brother, whose body Ankh was possessing.) This allows him to sacrifice himself by giving his last Medal for Eiji to defeat Maki.
Psych has one in Season 2, Episode 11: "There's Something About Mira", though it's a major part of the plot: Mira's fiancée, Jann, was a con man whose job was to romance Mira and have her marry him on a beach, then leave her at the altar while he and his partner robbed her family's winery. Unfortunately, Jann actually fell in love with Mira and couldn't go through with the plan, and doubly unfortunately, that leads to a Heel Face Door Slam...or something close to it.
Subverted on American Horror Story: Murder House. Tate is a total sociopath who has done terrible things. Then he falls in love with Violet and begins to turn around. However, when Violet finds out about the terrible things he's done she dumps him and the finally remorseful Tate is left alone. That ends up being his punishment for his behavior.
On Orphan Black, Delphine's love for Cosima leads her to betray the Neolutionists.
A human recruited to be a demon keeps his humanity by being reminded of his love for his mother.
Subverted in Cole's case. Initially his love for Phoebe leads him to do good, but it wasn't enough to make up for all the evil he had done. Absorbing a whole lot of demonic powers didn't help.
Once Upon a Time Tying into the overall theme of true love is the most powerful magic of all, this applies to both Regina and Rumpelstiltskin. Both of them have a certain amount of Love Makes You Evil as well.
Regina's love for her son Henry, and desire to be loved by him, is what lead to her redeeming herself.
The same motivations for Rumpelstiltskin for Belle and his son Neal.
Subverted for Jimmy Jacobs, leading to The Age of the Fall storyline in Ring of Honor. Jimmy fell madly in love with his manager, Lacey; after she eventually came around and told him that she loved him too, he crossed the Despair Event Horizon when he realized that her love didn't redeem him, or make him feel any less hollow, or magically turn him into a better person. Quite dark and heavy stuff, if you think about it.
Exalted gives us the Abyssals, the result of what happens when Solar Exaltations are corrupted to serve the cause of death by the Neverborn. There's just one small problem — those Solar shards still have bonded Lunar mates. And where an Abyssal usually picks up Resonance from engaging in "lively" activities (e.g., siring children, having sex with the living), they accrue none of it if they're with their Lunar mate. Oh, and since the Neverborn stripped out the Great Curse to make way for Resonance, if the Lunar manages to actually redeem the Abyssal back into being a Solar... they now have no psychological limitations that would directly drive them towards being nanners. Good show, dead gods.
Return of the Scarlet Empress also suggests that the Yozis will run into a similar problem with their corrupted Solars, the Infernals. Once their work gets underway, odds are likely that quite a few of them are going to rediscover their bond with the Lunars and turn their back on the Reclamation.
Two Pathfinder adventure paths exist, so far, that include romanceable, redeemable monster NPCS.
"Legacy of Fire" has Undrella, a harpy with a canonical Character Alignment of Chaotic Evil and the standard array of cultural quirks — terrible hygiene, sadism, appetite for humanoid flesh, and incorrigible flirtatiousness. It's even assumed by default that a PC will try and make friends of her (at least) and allow her to stay as a member of the restored village; when she reappears in the final adventure, she's actually changed her alignment to Chaotic Neutral.
A male protagonist can redeem Bastila more easily if they've fallen in love during the game. However, if you fail, she tells you that her feelings for you helped to hasten her fall to the Dark Side. However, if the male protagonist chooses to go Dark Side then him and Dark Side Bastila go on to become a happy little Sith couple, thus subverting the trope again.
Meanwhile, if a female protagonist turns to the Dark Side, Carth attempts to offer her a Last-Second Chance. The official version of the game subverts the trope at this point, as the PC can't be redeemed through Carth's love and the player's only options are to have the PC kill him herself or to allow Bastila to cut him down. A third option cut from the official release, however, would have allowed the PC to be redeemed.
The sequel also contains Visas, who starts as a Sith apprentice and ends up abandoning her Sith ideology if the player is light-side. This is an example of the Power of Friendship if the player is female, though.
Jolee's backstory subverts this trope. All the same, though, he maintains that love will save you, not condemn you; it's unrestrained passion that is dangerous.
Neverwinter Nights contains an example near the end with Aribeth. That scene is so similar to the one mentioned in the Knights of the Old Republic section that it seems like they just rewrote some of the dialogue to make it make sense in the Star Wars context, in addition to recycling the character.
Neverwinter Nights 2 has something like this with Neeshka if a male player has enough Influence with her in the ending sequence of the main campaign.
And in Mask of the Betrayer, love is the primary thing that gives people the strength to challenge the monsterous injustice of the Wall of the Faithless.
Jaffar in Fire Emblem 7 was a heartless killer until he met Plucky Girl Nino. His newfound ability to feel led him to rescue her from assassins and (if they get enough support conversations) eventually fall in love with her.
Averted in Breath of Fire IV. The Big Bad, Fou-lu, is beginning to nurse a soft spot for the girl that took care of him when he's injured. The girl, Mami, tried holding off soldiers from the Empire that are hunting Fou-lu just so he can escape, and gets captured herself. She's later used as a Sacrificial Lamb for the Nightmare Fueled Carronade, aimed at Fou-lu himself. Even before the cannon was fired at him, Fou-lu feels compelled to Kill 'em All more than ever...
Super Robot Wars Original Generation give Wodan Ymir Sanger Zonvolt's clone redemption in this manner. He originally was given the mission of protecting "Magus" as a way of tethering his persona and keeping him under control, but when he actually fell hard for her, his creator, Lemon, more or less told him to follow his own will and do what he thought was right. As a result, he does a Heel-Face Turn, joins up with Sanger, and together kick an EPIC amount of ass, with Wodan dying, but having the eternal respect of Sanger, the woman he loved protected (and Sanger taking up the mission to do so in Wodan's place per dying request), and he got to die as his own person, not just an enslaved construct.
But subverted for Morrigan. She sees this trope coming and backpedals desperately to stay a bitch.
The sequel viciously subverts this with Anders. As his sanity slips away and Vengeance corrupts him further and further, he pleads with Hawke to help him find the ingredients to create a potion that will separate him from his demon side. It turns out that the ingredients are actually components for a bomb, and Hawke's love has made him/her an unwitting accomplice rather than his redeemer. The same thing happens whether or not you romance him, but it hurts a lot more when it telegraphs and then snatches away this trope.
There's also Thane Krios, although he was aware of this trope before he meets Shepard, with his wife, who dies. Although it can be presented to him again, if a female Shepard decides to romance him.
Virgil from Xenosaga. What makes this interesting is that the person that made him have a case of Love Hurts (Febronia) is also the one that redeems him.
Count Bleck/Blumiere in Super Paper Mario actually ends up saving the world more or less thanks to this. After being reunited with his true love that he previously thought was dead, he pulls a Heel-Face Turn that is powerful enough to recharge the Pure Hearts and break the Final Boss's invincibility.
In the DS remake of Dragon Quest IV, beating the boss at the end of the Bonus Dungeon for the first time allows the player to obtain a Yggdrasill flower, which is used to resurrect Rose, the Big Bad's girlfriend. Bringing her to where Psaro was will not only cause him to revert back to his normal self after a nice flashback, but he will join your party.
Implied to be the reason why Richard Ames allowed Nastasha Romanenko to live through the operation as well as supply evidence against the Patriots in Metal Gear Solid 2's in-game novel: In the Darkness of Shadow Moses: The Unofficial Truth. Suffice to say, he also ended up paying for it dearly for it in the game when the Patriots decided to off him during the Big Shell Incident.
Played straight and subverted in Red Dead Redemption, as John Marston's love for his family ultimately leads him to evacuate his wife and son while the army guns him down, knowing that they won't be safe until it's over. Subverted in that Jack was corrupted by the time he spent with John, using the lessons he learned to become a man like his father, exactly what John died trying to prevent.
In Radiant Historia, the true ending of the game has the Big Bad Heiss finally accepting his place as the Sacrifice to spare Stocke, whom he truly loved as a nephew, the same fate.
In Dark Souls Eingyi was a nasty little joker, and is implied to be the reason why the swamp below Blighttown is poisonous. However, after meeting the Fair Lady and her saving his life at the expense of her own health, he happily resigned himself to serving the Ill Girl faithfully and carrying her eggs for her.
In Borderlands 2, Krieg the Psycho's motivation for joining the Vault Hunters? The sane side of him is in love with Maya, and thinks that she might be able to, one day, help him at like a normal person again, instead of a rampaging lunatic who constantly screams about riding bicycles made of meat.
Good!Krieg: Now tell her "thank you". Tell her that because of her, we might actually one day be able to act like a normal person again. (Beat) Krieg: (threateningly) I POWDERED MY COCKATIEL FOR THE RIBCAGE SLAUGHTER! (Another Beat, then Maya smiles at him.) Good!Krieg: (understandably surprised and delighted at the same time) Heh... Close enough.
Sacred: One of the Player Characters is a Dark Elf warrior who sets out with a bunch of mates to capture a sacrifice. They end up taking a Wood Elf (also a Player Character) prisoner, but as they're carting her back to her grisly fate, the Dark Elf ends up developing a bit of affection towards his newfound captive. He eventually betrays his comrades and sets her free, before becoming a full-blown Defector from Decadence.
Inverted and then played straight in Fate/stay night. Sakura, the heroine of Heaven's Feel, is possessed by the Bigger Bad when her confusion over Shirou returning her feelings makes her vulnerable to its influence. Played straight in that she gets better because Shirou and even her sister don't abandon her.
Kohaku in Tsukihime when you finally find out how messed up she is and then get the chance at her path. She almost lampshades it at the end of Hisui's route where she notes rather calmly that if Shiki hadn't confused the behavior of her in the past for Hisui now and given her back the ribbon she would have called off her plan.
In Yo-Jin-Bo, Ittosai is slowly redeemed by your love for him if you choose his path.
In the final part of Demonology 101 Isaac Jenner agrees to give up on what has been up until then his driving motivation, namely killing his older brother Gabriel in order to become the head of the Jenner family, as part of a deal with The Hero to save the life of Madeline, a demon whom he has fallen in love with. He keeps his promise, and does not bother the protagonists again (though he is still evil enough to get married on labor day weekend).
Arthur, King of Time and Space gives a glimpse of this in the science fiction arc when one strip follows Morgan down the years in her plots to tear Arthur's kingship apart. She eventually realizes that Arthur, Lancelot, and Guinevere genuinely love each other too much to be jealous at the love triangle, and asks herself, "I wonder what that's like?" Realizing that such love exists, and wanting it, will ultimately redeem her and bring her to join the Round Table.
Vriska in Homestuck got this in her conversations with John.
In The Order of the Stick, Haley's a complicated example. She was already Chaotic-Good— ish, as she put it, in obedience to her mother's Last Request. But meeting Elan, she found, made her able to be a better person.
Supervillainess Sahar in the Whateley Universe. She has a psychic ability to - once she's seduced a Psi - get so close that she can learn to copy that Psi's best 'knacks'. This makes her a ruthless femme fatale, until she falls in love with a mark, Zenith. She doesn't know how to handle that. So it takes a different kind of love - friendship - to get her to finish her Heel-Face Turn. And she gets Zenith back.
In Survival of the Fittest, John Rizzolo pulls a Heel-Face Turn when the lover he's been after the whole time, Emma Babineux, kisses him. Cruelly subverted when Rizzolo reveals he was just using that as a way to get Emma in a vulnerable position, and proceeds to stab her and then shoot her in the head.
One of the two big bads of The Delta State begins to develop feelings for Philip early in the series. It evolves from that to her intentionally not spotting him when she and Sven are looking for them, to her full on taking a shot for him and joining their side. Unfortunately it's pretty short-lived.
Gargoyles' David Xanatos started his lengthy path to redemption (or at least relative neutrality) after admitting his love for Fox.
His love of his and Fox's son Alexander also plays a part; after Goliath saves his son from Oberon, he becomes determined to repay the gargoyles for their help. While Goliath isn't willing to fully forgive or forget everything Xanatos has done up to that point, he is willing to give Xanatos the benefit of the doubt because knows first-hand that having a child can change a person for the better.
The Venture Bros.: Word of God has stated that The Monarch and Dr. Girlfriend are meant to be a subversion - they are still irredeemably evil even if they do have a loving, functional relationship. The fandom, however, tends to ignore their Moral Event Horizon moments and assume that this trope is in effect. It helps that said moments are usually absolutelyhilarious.
ReBoot. Matrix's love for AndrAIa is really the only thing that kept him from becoming a complete psychopath during the Time Skip in the games. Shes also the only person that can reliably get him to calm down when his short temper gets the better of him.
In the series finale of Spider-Man: The Animated Series, the Omnicidal Maniac Spider-Carnage regains his sanity for a brief moment when he meets the Uncle Ben of another alternate universe (all of the alternate universes have critical differences in Peter's life, in this one the big difference is that Ben didn't die). Sadly, the Carnage symbiote's grip on his mind is too strong, so he throws himself into another dimension separate from all others to keep his bomb from destroying all reality.
Although not "evil", Kimmy The Cheerleader in Sym-Bionic Titan was at first an Alpha Bitch and quite mean to Ilana. She was trying to persuade Newton to do her homework for her, but when he convinced her she's not as shallow or dumb as she thinks she is, she falls in love with him and ends up becoming his girlfriend. She starts to get along with Lance and Ilana after that as well.