- Horseshoes and Hand Grenades has Gentaro do one when Kengo didn't revive him. A being known as Ophiuchus resurrected him and brainwashed him to believe that his friends wanted him to die and left him to the hands of Ryusei Sakuta/Kamen Rider Meteor. Things get very bad.
- In The Keys of the Kingdom, Organization XIII consists of those who fell to the darkness and became Nobodies. This includes Emiya Shirou, Matou Sakura, and Takahata Takamichi.
- A major plot point in Out of the Corner of the Eye is Paco making a deal with Hsi Wu, becoming his host in exchange for the power to kill the man who killed his parents.
- In Respect, Yayoi Kise undergoes a turn from sweet, timid, crybaby artist to berserk supervillain due to severe guilt and Sanity Slippage.
- In Rise of the Villains the four major villains of the story, Teddy, Justin, Dimitri and John were protectors of the Multiverse until they went mad with power and now want nothing but to conqure the Multiverse.
- SAPR: Amber decides to join Salem's faction because she's afraid to be hurt in any way, and so figures it's best to just give the Relic of Choice to Salem so that she can just wonder off into the sunset with her enthralled 'friends.'
- In the Magical Girl Crisis Crossover Shattered Skies: The Morning Lights, an enraged Sayaka Miki willingly joins Dead End, the story's Legion of Doom, after regaining her memories of what Homura did at the end of Rebellion. Not even pleading from Mami and Kyoko can convince her that she's on the dark side, and she impales Kyoko to prove that she's Not Brainwashed and knows exactly what she's doing and where she stands. All of which takes on an entirely new context many chapters later, when she's revealed to have been a Fake Defector from the beginning, having run a long-term secret plan to earn Dead End's trust so she could serve as The Mole.
- In With this Ring... (Green Lantern), Guy Gardner joins Star Sapphire and Hector Hammond out of resentment towards Hal Jordan.John Stewart: So what made you side with the enemy, Gardner? I never liked you, but I thought you had more class than that.
Guy Gardner: The enemy I've got is Jordan. I lost a woman because of him. I lost a mind because of him. Now I've got a new mind... and I like it just fine.
- Happens in Some Semblance of Meaning, when Phlox allies herself with Vale, Fen, and Lark, only to attempt to murder them in their sleep. Thankfully, this fails. However, Vale thinks to herself later on that, in spite of her actions, Phlox wasn't really evil; it wasn't the right thing to do, but "this is the Hunger Games. We all do things that we don't want to do—like killing and stealing and betraying—because let's face it: every single one of us wants to escape this sadistic prison the Gamemakers set up for us, to make it home alive. To our friends and families. For that, we would do anything."
- Tambryn in Claustrophobia started off as a nice, well-liked character. Then she went off the deep end and tried to crucify her ally and district partner Maxim, who was only thirteen.
- The entire plot of Queen of All Oni is kicked off when Jade's Superpowered Evil Side, the Queen of the Shadowkhan, is reawakened, setting Jade up as the Big Bad.
- In the second half of the story, Jade brainwashes Viper into being her new Samurai Khan General, Hebi.
- Bond’s supposed informant in Diamond's Cut, a fan film.
- Don't Haze Me is a Kim Possible one-shot where Kim goes down the slippery slope to becoming a villain. Instead of Shego undergoing a Heel–Face Turn and joining Kim like in other Kigo fics, Kim takes a Face–Heel Turn and joins Shego. Kim becomes a Well-Intentioned Extremist who wants to make the world better by killing the "bad guys", including Drakken.
- Maylu in MegaMan NT Warrior fanfiction, Maylu's Revenge.
- Truth and Consequences begins when Ladybug, of all people, makes a deal with Hawkmoth that includes giving him the Ladybug and Black Cat Miraculous. She has legitimately good, if ultimately selfish, motivations: protect Gabriel Agreste's son and employees from the fallout should his identity as Hawkmoth become public knowledge. However, things rapidly begin spiraling out of her control, and she's forced into increasingly morally dubious choices in pursuit of her goal. She ultimately winds up making a Heel–Face Turn back, but by that point, Hawkmoth has both Miraculous, and Paris is in worse danger than it ever has been.
- In "Watch Me Burn
," Chat Noir ramps up his harassment of Ladybug at the expense of his duty to protect Paris, including deliberately endangering civilians to blackmail her into saying she has feelings for him. After one of these incidents goes viral, tanking his reputation, he stops even pretending to care about the city, deliberately sabotaging Ladybug during fights and threatening to let Hawk Moth win unless she gives him what he wants.
- Mortal Kombat: Desperation: The Big Bad of this fanfic, Raiden, becomes this after he Came Back Wrong. He starts out as a Well-Intentioned Extremist with a bit of the Knight Templar and He Who Fights Monsters mentalities, but as the plot progresses, he becomes increasingly homicidal and sociopathic to the point of alienating his ex-allies when he refuses to reconsider his invasion of Outworld and when he crosses the Moral Event Horizon by mass-zapping the Lin Kuei to near-death.
- Entropy: The Fate of the Hero System: Momo Yaoyorozu and Melissa Shield, as well as the entirety of the former Class 2-A. They become the Fallen Heroes Alliance.
- Rise of the Last Villain: Shoto chooses to join the League of Villains after they promise to make Endeavor pay for what he did.
My Little Pony: Friendship Is Magic
- Many characters in Equestria: A History Revealed end up pulling this trope.
- King Sombra was initially an academic scholar with a love for learning and writing books, and turns into the snarling evil villain of canon, hinted to be because he was driven insane by the crystal ponies annoying him too much with their inane requests.
- Same as in canon, Princess Luna becomes Nightmare Moon, and provokes the Equestrian Civil War out of jealousy of her sister. This is also expanded upon in the fic to also be due to her massive unpopularity and her thinking that Celestia no longer loved her any more.
- In a smaller example, the pegasi war hero Thunderhide also joins Luna's side after losing faith in the Celestia-Equestrian system, and feeling that his bretheren's sacrifices in the war were in vain.
- The plot of Inner Demons is driven by a prophecy that Twilight Sparkle will succumb to the darkness in her heart and become the "Queen of Darkness", who will take over all of Equestria. This happens within half a dozen chapters, and Queen!Twilight proceeds to take up the mantle of Big Bad for the story.
- Trixie starts out the story as The Atoner, who wants to learn true magic from Twilight in order to make up for her boastful and arrogant past. When Twilight turns, she talks Trixie into helping strike at the other Elements of Harmony... then Trixie sees how powerful Twilight is, and performs a full turn herself in order to convince Twilight to keep her.
- Sweetie Belle and Scootaloo, join up due to a combo of More than Mind Control and not seeming to be fully aware of what's happening.
- In the Pony POV Series, Fluttershy suffers a severe Break the Cutie and falls victim to the same corruption as Nightmare Moon, transforming into the Big Bad of the story arc, Princess Gaia/Nightmare Whisper.
- There's also the Dark World versions of the Discorded Mane Six, who Discord uses as his Co-Dragons for a thousand years until Cadence's spirit helps Twilight remember her true self, beginning the eventual Heel Face Turns of all of them.
- The Nameless Passenger turns out to be a version of Twilight who turned into a vengeance crazed Nightmare seeking to punish Discord for all eternity via "Groundhog Day" Loop, no matter how many lives she may ruin or erase in the process.
- Neon Genesis Evangelion: Genocide: When Ritsuko is sick of being Gendo's tool, she allies herself with Kluge to bring her ex-lover down.
- All Mixed Up!: The Big Bad of the story, Mariana Mag, used to be known as Ocean, an Investigation agent of Precinct 13579 whose expertise was language instead of mathematics. However, when Oprah forced her to learn and love mathematics while dropping language entirely, she rebelled and was eventually fired from Odd Squad. With the loss of her Proportional Aging abilities, she grew up into an adult and inherited her father's aquarium from him once she graduated college, while also keeping her revenge against Odd Squad hidden until the right time.
- Viva La Vida: Played with in three ways regarding Olive turning to evil.
- The Deconstruction. Olive gradually makes the switch to evil as the story progresses. While agents turning to the "odd side" is common enough for therapy programs to be created (at least in Precinct 13579) and they can take advantage of the programs if they feel themselves slipping, Olive, who has full knowledge of the assistance available to her, doesn't take advantage of them. Instead, she has to battle her emotions and feelings as she struggles with why she enjoys being around Odd Todd instead of being disgusted with him like everyone else.
- Played straight in the Bad Ending, where Olive completely goes to the "odd side" and joins the Todd Squad permanently right after she performs a Redemption Rejection towards Otto.
- Subverted in the Good Ending, where she has a Heel Realization and decides to quit the Todd Squad, heading back to Precinct 13579 and owning up to what she did.
- In Pokémon Strangled Red, Steven will do anything to bring his Charizard back, even using an unnatural power and killing his own brother.
- May in Sunny With A Chance Of Thunderstorms betrays Brendan and joins Team Aqua.
- In Red Poison, this happened in a parallel reality in which Jason switched sides out of jealousy of Tommy.
- Children of Remnant: Downplayed; to the audience, it was clear from the beginning that Winter was on her father's side over Ironwood's, but since James didn't see that, her suddenly usurping him from power and all but invading Vale comes across as a betrayal.
- Sherlock Holmes in Children of Time, as the combined end result of a Break the Cutie experience and a Sadistic Choice. He later tells Beth Lestrade that he was "curious". Her unconditional love and Heroic Sacrifice force him to truly understand what he's become and what he's done to her.
- In Syxx Fox's Rogue Fox Series, the breakup with Krystal and the abandonment of his friends who had little time in continuing the mercenary life has our beloved protagonist falling from grace after depression leads to severe mental and emotional turmoil, corrupting him into his worst enemy. As his resentment towards everyone, most predominantly Krystal, grows into pure hatred, Fox becomes more and more dark and still is; even after the two sequels following his defeat at the hands of the woman he once loved (and now utterly hates abhorrently) Fox, renamed "Syxx", makes a long-range plan to claw back power and enact revenge on his former teammates and love interest in the potential future. With the addition of killing his father when the time came.
- The whole first installment of the Son of Suns Trilogy is an extended Face–Heel Turn for Luke Skywalker.
- Total Drama Genesis: After returning at the merge Diamonique acts noticeably eviler, voting off Cynthia despite being alliance members and friends because it will benefit her.
- Unbreakable Red Silken Thread: Gwen isn't the shy reserved Goth girl she once was after her relationship with Duncan.
- Where We Don't Belong: Cammuravi is now a member of Torna and is thus fighting to destroy the world of Alrest, which takes Mio and Sena by complete surprise.]]
