Zuko from Avatar: The Last Airbender does a beautiful FaceHeel Turn in the season 2 finale. Actually, he does a FaceHeel Turn, then a HeelFace Turn, then another FaceHeel Turn, then a few somersaults, ending with another HeelFace Turn. Say that three times fast. His first FaceHeel Turn came when he was 12. An innocent, well-meaning boy gets half his face burned off by his loving father. After this, he hunts Aang ruthlessly for his "honor." His second, and most exemplary FaceHeel Turn, came at the second season finale. After starting fresh in Ba Sing Se, Zuko has to choose between his honor and his Uncle. After a blatant "I Hate You" speech, he attacks Aang and returns home a hero, at least until his final HeelFace Turn.
Book 4 gives us two more Face-Heel Turns. Kuvira went from Suyin's protegée to a power-hungry dictator who claims to want to bring peace back to the Earth Kingdom but really seeks control, Baatar Jr. who followed Kuvira and took a massive level in Jerkass in the process. Zhu Li also seems to perform a Face-Heel Turn in "Enemy At The Gates", pledging loyalty to Kuvira after finally getting fed up with Varrick for his poor treatment of her, but "Operation Beifong" reveals her to actually be a Fake Defector, who deliberately got close to Kuvira so she could sabotage her superweapon.
Harvey Dent going from the respected attorney he was to the dangerous criminal Two Face in Batman: The Animated Series is done as a serious Tear Jerker. He had already been an established and likable character beforehand who suffered from anger issues until a disfiguring injury pushed him over the edge and made his psychotic alternate personality become dominant, and Batman has never once stopped regretting being unable to save him or given up on swaying him back to being a good guy.
Mirroring Harvey is Detective Ethan Bennet in The Batman who, due to an abusively Bad Boss, being "clowned with" by The Joker, and a harsh dose of mutagenic gas, becomes the amorphous shapeshifter Clayface. Unlike Harvey he struggles much more with morality, initially only being out for revenge, trying to live a normal life, and even attempting to use his powers for good, before his degrading sanity makes him decide Being Good Sucks and he'd rather be a criminal. He eventually does get better for good, helping Batman take out the much more dangerous Clayface II and being cured in the process.
Albedo from the Ben 10 franchise did this. In a flashback from when Ben was 11, he was allied with Ben and Azmuth, and was at least a respectful, decent guy at the time. From Ben 10: Alien Force onward however, he is a major enemy of Ben Tennyson, for he did turn evil during the Time Skip between shows.
There's also Captain Nemesis/Overlord, who made his debut in Ben 10: Ultimate Alien. He was a famous hero before Ben even got the Omnitrix, and Ben was a huge fan of him. Then Ben ended up stealing his fame, he became jealous, and he ended up turning to villainy, forcing a Sadistic Choice onto Ben. Needless to say, Ben was not happy.
"Turf Wars", the last episode of the 2006 revival of Biker Mice from Mars, revealed that Harley, Vinnie's girlfriend and one of the Martian freedom fighters who rebelled against the Plutarkian invasion from the original 1993 series, became disfigured while trying to flee from the Plutarkians and sided with the Rats because she believed the Biker Mice abandoned her. The Biker Mice did not abandon her, and were in fact aghast to see an old ally become their enemy.
A Flashback episode of GoBots revealed that Cy-Kill was once Leader-1's best friend, but frustration over the leadership of Gobotron caused Xy-Kill to join the rebellion (and eventually lead it). In fact, the name of the episode where this is told is "Et Tu, Cy-Kill?"
Codename: Kids Next Door had an agent, Numbuh 274, who had appeared in a couple Season 1 and Season 2 episodes become a villain from Season 3 onward after turning 13. However, it turned out that he'd been working for the good side all along in the penultimate episode of the show.
The fourth-grade class president also began as a heroic character but, after the KND works to ensure his safe election, it's revealed that he's actually their enemy. The exact nature of his FaceHeel Turn is something of a mystery: while his blank eyes, Creepy Monotone and Father's Evil Laughter during his FaceHeel Turn implies that he'd been brainwashed, later episodes establish him as not only a perfectly willing heel, but one whose agenda has little connection to Father.
Denahi in Brother Bear, after he thinks a bear killed his brother Kenai (in reality, his brother Kenai was turnedinto a bear). He spends most of the film on a Roaring Rampage of Revenge trying to kill Bear!Kenai until the very end where Kenai is transformed back right before his eyes.
Ed, Edd n Eddy parodied this in Ed, Edd n Eddy's Big Picture Show when, after a misunderstanding caused the other kids to beat up Captain Melonhead (Jonny) and Splinter the Wonder Wood (Plank), they became The Gourd and Timber the Dark Shard to get revenge on the cul-de-sac. Too bad the movie was over.
A fictional pro wrestling example. In one episode of Futurama, Bender becomes a popular robot wrestler named Bender the Offender. After his popularity fades, management turns him into the Gender Bender.
Demona in Gargoyles, who became obsessed with committing genocide against humans after her clan was massacred.
Green Lantern: The Animated Series: This happens to Aya after she shuts down her emotions and takes over the Anti-Monitor's body, becoming a Knight Templar bent on eliminating all organic life to the pointat she goes back in time to rewrite the creation of the universe. She comes to her senses when she accidentally nearly kills Razer.
In Iron Man: Armored Adventures, Gene Khan does this in spectacular fashion during the second half of the season finale. Given that the writers had spent the entire first season making this character friends with the hero, humanizing him and integrating him into the True Companions, this is actually fairly stunning, especially given that this show loves sympathetic villains being redeemed the way it does.
One could argue that Gene made his original turn at the start of the series, when he threw down his stepfather and took the role of Mandarin for himself. Outing himself to his friends after they out themselves as Team Iron Man just sealed the deal.
Whitney Stane has often straddled the line between good and evil ever since she's donned the identity of "Madame Masque", going from Unlucky Childhood Friend towards Tony to Stalker with a Crush to him, with murderous tendencies towards her father, to redemption, to a brief Anti-Hero stint as Madame Masque, and Laser-Guided Amnesia that led her to be Demoted to Extra. Her heel turn is permanently established in "Iron Monger Lives!", where she remembers Tony's identity as Iron Man and her time as Madame Masque. She returns as Masque to hatch an elaborate scheme in order to ruin Tony's life and kill his friends and family. By the time the episode ends, she seems to be continuing her villainous persona and her vendetta.
In Kim Possible, Shego used to be a superhero before becoming a villain. Various other characters have short heel turns, often for a single episode, including Ron at least twice.
Before then there was Princess Luna's descent into Nightmare Moon during the backstory.
The season 7 finale villain, Stygian, was once the Jimmy Olsen to Starswirl the Bearded's Superman. It was over a thousand years ago that he desired to become a hero like Starswirl and the other Pillars of Equestria, so he borrowed their artifacts of power without asking, intent on creating copies. Unfortunately, the Pillars decided that the theft alone was proof that Stygian was evil, and harshly rejected him. He then made his way to the Well of Shade, embracing the dark powers within and becoming The Pony of Shadows.
The Oh Yeah! Cartoons short "Thatta Boy" has Thatta Boy's sidekick Polly decide to become a villain near the end after being disillusioned in the career of crime-fighting. Much to her partner's frustration, her first villainous act is to set free all the criminals he captured.
Princess Entrapta eventually joins forces with the Horde in season 1. Not out of malice or hatred; even though her friends left her behind in the Fright Zone, she bears them no ill will. No, she turns because her only motivation is For Science! and the Horde give her a lot more options for experimentation. The fact that her experiments are hurting people never seems to bother her, if she even realizes that's a thing.
Hordak's second-in-command Shadow Weaver was once a mage in Mysticor known as Light Spinner. After dabbling in dark sorcery, she was cast out, and joined the Horde. Season 2 shows that even as Light Spinner she was prideful, ambitious, and deceitful, but she used to have good intentions.
Barnacle Boy in Spongebob Squarepants, however it's only for one episode over the fact he wasn't allowed to have an adult-sized Krabby Patty.
Star Wars: The Clone Wars: Senate Commando Captain Argyus (in Season 1), Clone Sergeant Slick (in Season 1 as well), Pong Krell (in Season 4), Barriss Offee (Started out as a heroine in Season 2, becomes a baddie in Season 5).
The Silver Surver on The Super Hero Squad Show was so corrupted by the Infinity Sword, that he turns his back on Galactus and even tricks Thanos into giving him the complete Infinity Gauntlet. It's rare for a Face Heel Turn to pull a Starscream, but Silver Surfer pulls it off well and becomes the new Big Bad till the end of the series.
In the second TMNT series, the real Oroku Saki was originally one of a group of warriors that defeated a powerful demon. However Saki absorbed the power and became the Tengu Shredder.
Vernon Fenwick in the first TMNT series, to a lesser degree. He is originally, for the most part, is an ally to the Turtles, however, at the beginning of the eight season (the first of the infamous Red Sky Era), after the Channel 6 building is destroyed by Shredder, Vernon then sides with Burne Thompson in seeing the Turtles as menaces to society, as they weren't able to save the building in time (and Shredder blew it up because he believed they captured Krang).
Gibbs from [adult swim]'s Titan Maximum. His Face-Heel Turn in the first episode generates the conflict for the rest of the series. Unfortunately, it seems that he's perhaps the only character with any shred of competence.
Lightning near the end of Total Drama Revenge of the Island after accidentally giving Cameron the win in "Eat Puke and Be Wary", upon which he not only Took A Level In Jerk Ass, but also he replaces Scott as the Big Bad.
Dave in the Pahkitew Island finale, after finding out that Sky is already in a relationship, upon which his Sanity Slippage gets even worse.
Dinobot in the Transformers: Beast Wars episode "Maximal No More." He does a HeelFace Turn back at the end, though, upon realizing that just because everything's going according to Megatron's plans doesn't mean that Megatron isn't a dangerous madman.
This happens to most of the Maximals at one point or another.
In the episode "Double Jeopardy", Rattrap seemingly defected to the Predacons, though it was quickly revealed to be a ruse. The same episode has Tarantulus subjected a Maximal protoform to re-programming, creating Blackarachnia.
A couple episodes later in "Gorilla Warfare", Optimus was infected by one of Scorponok's viruses, turning him into a nigh-unstoppable beserker that attacked Maximals and Predacons indiscriminantly.
The episode "Dark Designs" saw Megatron kidnapping and brainwashing Rhinox- however, Evil!Rhinox was a bit too good at being a bad guy, and nearly usurped Megatron's control of the Predacons. So he changed Rhinox back, and the Maximal was able to escape.
This returns in the sequel series, Beast Machines. Rhinox's spark was used to make the Vehicon general Tankor. Even after he regained his memories of who he used to be, Tankor decided to remain evil, and was scarily competent. He did later express regret for his villainous actions, however.
Tigatron and Airazor, technically speaking- when they were merged by the alien Vok into Tigerhawk, their sparks were not originally in charge of the resultant body. But it was still thier body, and the Vok used it to attack both Maximals and Predacons, so...
Silverbolt was a Maximal protoform who was mutated by transwarp energy, and briefly persuaded to join the Predacon cause after awakening.
In Transformers Armada, the Autobot Wheeljack used to be Hot Shot's best friend. Until one particular battle, where Wheeljack became trapped under rubble; unable to lift it alone, Hot Shot left to get help, but was kept from returning by an Autobot commander. Instead, Megatron found and rescued Wheeljack, prompting him to switch sides.
Blackarachnia in Transformers Animated pulls her FaceHeel Turn during the (mainly) flashback episode "Along Came A Spider". We discover that when they were in the Autobot Academy, Elita-1, Sentinel, and Optimus went to an organic planet, even though it was forbidden, and they encountered giant spiders. Escaping from the spiders, Elita used her download power to borrow Optimus's grapplers, but ran out of time, falling into a pit full of the spiders when Optimus failed to catch her with his other grappler. She attempted to use her download power on the spiders, but, the spiders being organic, it turned her into a mutated part-organic, part-mechanical Transformer with one of the spiders as her alt-mode. The new signal type made her impossible for Optimus to pick up on his radar, and led him and Sentinel to believe Elita was dead, and Blackarachnia to believe they abandoned her, and she joined the Decepticons.
Thanks to the IDW comics and Transformers: Prime, this has become an integral part of Megatron's backstory. Once he was a well-meaning revolutionary, but he came to believe that Violence Is the Only Option, and became an even worse tyrant than the bots he fought against.
Transformers Prime also has the Orion Pax arc- after using the Matrix to defeat Unicron, Optimus loses some of his memories and reverts to being the archivist known as Orion Pax. Unfortunately, his old friend Megatronus was right there, and convinced him that the Autobots were villains responsible for both the war and Cybertron's destruction. Thankfully, Orion eventually uncovers the ruse.
This is also the backstory of The Fallen. He was once a member of the original Primes, but eventually became evil. Whether it was through the influence of Unicron, Knight Templar tendencies, or the accidental murder of his beloved.
Riven in season 1; he grows frustrated with the Winx and his fellow Specialists, and eventually abandons his group, starts dating Darcy, and feeds the Trix information about Bloom. In the 4Kids Dub, he's brainwashed. He went evil again in The Secret of the Lost Kingdom, but that was due to Mind Control.
Bloom twice in season 2 serves Lord Darkar, but that was because of Mind Control.
Aisha in season four. She join in the Nebula's group to kill the Wizard of the Black Circle and avenge Nabu's death. It didn't last long, though it was still pretty shocking.
Raimundo in the season one finale of Xiaolin Showdown. Shortly into season two, he reverses it and returns to his friends, but not without suffering some consequences and trust issues.
Omi undergoes an involuntary turn after coming back from the yin/yang dimension, leaving his good side behind.
Aqualad/Kaldur'ahm in season two of Young Justice, after he becomes embittered at his former friends for letting Tula die during a mission gone wrong and his own mentor, Aquaman, for hiding the truth about his real, supervillain father, Black Manta. Kaldur ends up joining him.
Turns out he only faked his turn and is deep undercover for the Team.
Quacky the Duck on T.U.F.F. Puppy. He used to be a happy, funny TV show host until his show got cancelled by the network president, forcing him to turn evil and get revenge by destroying all television.
An in Universe example seemingly appears in the first episode of Scooby-Doo and Scrappy-Doo. The Blue Scarab, a comic book super hero, appears to have come to life and is going on a crime spree (Much to the entire gang's confusion and Scrappy's dismay.) Of course, as per usual Scooby fare, it was just a guy in a suit, but until the end the trope was played entirely straight.
The guy in a suit may still qualify. Howard Gruger worked for Jerry Sloan, the creator of the Blue Scarab comics as his assistant, but became tired of being an unknown assistant and wanted to create his own comic book character, so he disguised himself as the Blue Scarab and committed crimes so as to destroy the Scarab and Sloan. Of course, Gruger could have just asked Sloan if he could leave so he could create his own comic book character instead of trying to destroy Sloan and the Blue Scarab, but he might also have been jealous of Sloan's fame..
This was loosely based on issue #24 of the Gold Key Scooby comic story "Mark of the Blue Scarab." The perp in this case was a comic book scripter who thought he was a better writer than Sloane was an artist, so he goes about terrorizing Sloane. Both the comic and the animated episode were written by Mark Evanier.
In Voltron: Legendary Defender, Zarkon and Honerva went from being heroes to villains due to them being exposed to Quintessence, which initially resulted in their deaths before being brought back to life and ultimately becoming the initial Big Bad Emperor Zarkon and Dragon-in-Chief Witch Haggar respectively as a result.
In Tangled: The Series, two different characters betray Rapunzel. The first is Varian after Rapunzel fails to keep her promise to help his father who is encased in a magical amber. The second is Cassandra, who after learning of her true heritage, being Mother Gothel's daughter, steals the Moonstone out from under Rapunzel's nose, takes its power, amd abandons the party.