In The Incredibles, Buddy Pine's career as Incredi-Boy is short-lived when he is rejected by his idol, Mr. Incredible. But Mr. Incredible learns the hard way that some people are Not Good with Rejection when he goes up against Syndrome, who is actually Buddy all grown up and out for revenge.
In Toy Story 3, Lots-o'-Huggin' Bear was once a kind-hearted toy who dearly loved his owner Daisy. Then he was accidentally abandoned by her, and this made him into an abusive, nihilistic tyrant.
Films — Live-Action
Baahubali: Queen Mother Sivagami starts off as a kindly queen protecting her people and her sons, but she evolves into a cruel and capricious woman due to Bhallaladeva's manipulations and her hatred of Amarendra's wife Devasena. She does have a Heel Realization after having Amarendra killed and sacrifices herself to save his son.
Kid Culprit in Beyond the Lights. In the beginning, he's kind of annoying and not exactly Mr. Sensitivity (he sexts his girlfriend right after she tries to take her own life, for one). But he's basically all right. After Noni breaks up with him (because she's started to fall for Kaz and realizes that he's the one she wants to be with), however, he basically sexually assaults her on stage during their BET Awards performance (doing everything from shoving her head into his crotch to literally trying to rip her clothes off), practically brags that he's been cheating on her, claims he dumped her and calls her a bitch, a freak and his "sloppy seconds" (as he grabs his crotch), all on national TV.
BloodRayne: Katarin betrays Brimstone after Rayne's accepted in their ranks, whom she distrusts. She murders other Brimstone members, seeking the Heart for her father, a vampire, whom Katarin thinks can be the lesser evil versus Kagan.
Emperor (2020): Downplayed. A preacher gives Shields refuge and treats his wounds despite how he's obviously one of John Brown's men, but he does acknowledge that Shields is hiding in the church when McCabe and Grady show up and offer him a choice between part of the reward or getting killed if he resists them. Since they already know Shields is hiding there, the preacher's acquiescence doesn't mean too much and may not be too wholehearted, but he doesn't make a Heroic Sacrifice rather than aid the slave catchers either.
Fantastic Beasts: The Crimes of Grindelwald: Queenie Goldstein defects to Gellert Grindelwald's side because she is swayed by his promise of being able to create a world where she and Jacob Kowalski can openly pursue a relationship.
Riley in Fast & Furious 6 is a straight example, defecting from the heroes to the villains. Actually, she was never on their side; she was The Mole planted by Owen to spy on the team.
Dom supposedly betrays his team in The Fate of the Furious. Turns out he never betrayed the team. He was blackmailed to protect his infant son's life.
Alec Trevelyan of GoldenEyeseems to be like this during the reveal of him being Janus, but as it turns out, he's been plotting revenge against the British government from the beginning for the betrayal of his family, who were Lienz Cossacks that the British sent back to Stalin at the end of World War II.
Terri in Hellraiser III: Hell on Earth. Feeling betrayed by everyone, Terri chooses to join Pinhead, and becomes a Cenobite.
The Velociraptor pack in Jurassic World have been trained and raised by Owen Grady as something like hunting dogs, hunting down the Indominus rex. However, since the hybrid has raptor DNA, she convinces the raptors to turn on their alpha and the humans in general. They do, however, switch sides only for Charlie to get blown up, and Delta and Echo get killed by the I. rex herself.
Arthur became The Mole for Valentine sometime prior to the film taking place.
After being kicked out of the Kingsmen training program, Charlie and his family join Valentine's plan, being some of the chosen few who will be spared from the upcoming genocide.
Kingsman: The Golden Circle: Whiskey is working against Statesman and Kingsman's efforts to get the antidote to Poppy's tainted drugs. Not because he's working with her (he's not), but because he happens to hate drug users enough to want them all dead for personal reasons.
When Vic makes his first appearance in The Man from Laramie, he seems like a fairly reasonable guy despite being on the side of the tyrannical Waggomans. By the end of the film, he's responsible for at least one murder, another attempted murder, and we even find out he's ultimately (indirectly) responsible for the death of the protagonist's brother which basically kickstarted the whole thing.
Over the course of Mean Girls, the friendly and good-hearted Cady usurps Regina's position and becomes the mean-spirited Alpha Bitch of the school. She goes through a subsequent Heel–Face Turn when she's confronted with her actions. Also, Jo in the sequel.
MonsterVerse: Emma Russell in Godzilla: King of the Monsters (2019). It looks like Godzilla has turned against humanity in Godzilla vs. Kong, but it turns out he was Good All Along. Both those films' novelizations have also featured a couple of Monarch's operatives aligning themselves with either film's human villains (one due to agreeing with Emma Russell that the Titans should be free, the other for money) respectively.
The trope image gets lampshaded in Muppets from Space where Rizzo the Rat tries to plead with Hulk Hogan by saying "What will your fans think?" The response: "Hey, I'm a bad guy now."
The Omega Code: Cassandra chooses to take Stone's side in the final confrontation.
Theodora in Oz the Great and Powerful starts off on the side of Oz, believing him to be the prophesied wizard, but due to the machinations of her sister Evanora, she ends up becoming a Wicked Witch.
In Pixels, Pac-Man, good guy in the games, turns into one of the antagonists the heroes must face.
David "Rooster" Fisk in Righteous Kill, who lost his faith in legal justice due to a child killer getting Off on a Technicality thanks to Turk's actions, leading Rooster to become a vigilante Serial Killer.
Rise: Blood Hunter: After becoming a vampire, Tricia becomes evil. It's left unclear exactly why, but a combination of the transformation affecting her, mental trauma due to what she undergoes, or having a sense of belonging with the head vampire Bishop all seem possible.
Scrappy from the first live-action Scooby-Doo movie. He went all evil because the gang kicked him out years ago and wanted revenge by taking over the Earth with demons. Though the flashback to him marking his territory on Daphne seems to indicate he was always a jerkass, and we just didn't know it.
Part of the backstory of Mother Malkin in Seventh Son. She was a good witch until Master Gregory married another woman, driving her to submit to the forces of darkness and become queen of the evil witches.
Anakin Skywalker of the Star Wars saga, who turned his back on the Jedi to become the ruthless Darth Vader.
A lesser-known occurrence is when Count Dooku left the Jedi Order and became a Sith. Also, many Jedi in the pre-Disney Star Wars LegendsExpanded Universe.
In Return of the Jedi, Palpatine tries to get Luke Skywalker to do this, to give in to the rage he's feeling towards Vader and become his apprentice on the Dark Side. It doesn't work out.
In The Force Awakens, Kylo Ren was Luke's pupil and nephew, formerly called Ben Solo, in the new Jedi Order, but he gave in to the Dark Side and exterminated the new Jedi, driving Luke into hiding as the last known Jedi Knight.
In the fifth Terminator film, Genisys, John Connor is turned into a Terminator and is sent to kill both his mother Sarah Connor and his father Kyle Reese.
While Kevin Flynn remains a protagonist throughout the entire TRON franchise, his program Clu turns HARD in TRON: Legacy. The comic book, however, makes it arguable as to whether Clu was ever a Face to begin with.
Sentinel Prime in Transformers: Dark of the Moon. He takes this up a notch by seizing Megatron's position as the new leader of the Decepticons.
In Undercover Brother the title character turns his back on the Brotherhood after he falls in lust with Penelope Snow. Luckily, Sista Girl brings him back to his senses.
Elysia in Warriors of Virtue. Prior to the movie's events, her brother was killed by Yun, which led to her developing a grudge towards him and her turning to Komodo. During the movie, though, she does develop a soft spot for him and thinks of him as a brother. It's this that leads to a near Heel–Face Turn, changing her back to what she was before. Unfortunately, her decision to protect Ryan ends in Redemption Equals Death.
The Yakuza: Tanner eventually turns on Kilmer and puts out a hit on him in exchange for Tono sparing his life.
