A Pupil of Mine Until He Turned to Evil

Well, at least his son won't turn out as bad.

You talk too much, as always. I always liked you. Those dumb jokes. Your eccentric poems. But no more. It's different now. You're not my teacher. You're nothing. Just a tool. A tool to make Muhyo feel pain.

—Enchu, Muhyo and Roji's Bureau of Supernatural Investigation

A mentor once had a student, who used to be a good person, and perhaps the mentor's most promising and skilled student ever. But that student had a flaw, a seed of villainy that just needs a trigger. Usually it's arrogance about his/her skills or impatience at not being taught the final skills that the mentor has yet to teach. Then the student becomes a villain, even the Big Bad.

Sometimes the mentor is the protagonist, and will have to take the student down in the story. But far more often the protagonist is another student the mentor takes on.

Compare Deceptive Disciple (when the student was evil the whole time, just pretending not to be), Fallen Hero, Merlin and Nimue, and The Paragon Always Rebels.

Do not put examples unless it was clearly stated the pupil was good beforehand. If it turns out the pupil wasn't, that's Deceptive Disciple.

Examples of A Pupil of Mine Until He Turned to Evil include:

Anime and Manga

In Alice 19th, Darva was originally Master Lotsuan's disciple.
In Soul Eater, Asura was Shinigami's apprentice until he went insane and threatened to destroy the world, and had to be sealed away.
Naruto offers both Orochimaru and Sasuke. Orochimaru was considered the village's best ninja when he was in Konoha, then got fed up with things and left for his various evil deeds. Sasuke was lured away by Orochimaru.
- There's also Nagato, also known as Pain, a former apprentice of Jiraiya. More of a Well-Intentioned Extremist, but he still qualifies because he puts such a heavy emphasis on the "extremist" part. Especially since his 'good intention' is to invent a chakra nuke and use it so that everyone will be too afraid to go to war for a few generations.
- The Third Hokage notes that he always suspected something was amiss about Orochimaru, so it's unclear when exactly he turned evil, even if he was evil before he left the village (around the time the Third Hokage chose Minato for the position of Fourth Hokage largely because Orochimaru was untrustworthy.)
  - Apparently a younger Orochimaru was 'good' enough to cry when Dan died. Wherever he started, he did a lot of falling.
  - He was also always Hiruzen's favorite, just to drive in the Sasuke parallel, so the old Professor can't have suspected too much.
    - - He suspected, but he thought that since it was a time of war a kid with malice and ambition might end up being useful, so long as he could be guided and controlled. He later admitted he let his pride blind him. Oro only left the Leaf after commiting a series of brutal murders and human experiments, and being caught, so he was definitely bad before he outright defected.
Enchu for Chief Page in Muhyo and Roji's Bureau of Supernatural Investigation, who also taught Muhyo.
Fullmetal Alchemist pulls a variation, in that Homunculus kind of mentored Slave #23 from the day he gave him his name...but recognized Van Hohenheim as 'his father.' Given his entire identity is later molded around the fatherly role, this concept is clearly significant to him.
- Thus Hohenheim's part in the finale manages to combine The Obi-Wan role here with Calling the Old Man Out. And Ed gets to be the third party and be 'the lead' when they off his big brother.
- The first anime played with the trope in that Ed's master Izumi Curtis had trained with an old woman named Dante, who turned out to be the villain.
In Dragon Ball, the Turtle Hermit states that the Crane Hermit used to be a fellow student of the mighty Mutaito, who underwent a Face Heel Turn after seeing the might of Piccolo Daimao, having apparently decided that Being Good Sucks.
In episode 10 of Samurai Champloo, the gang meets up with a friendly monk who turns out to have been a former martial arts teacher who once taught Shoryuu, the villain of the episode. He had encouraged Shoryuu to travel and hone his skills, but this went horribly wrong, when after learning an amazing technique, Shoryuu became obsessed with proving his greatness by challenging and killing any martial artist he could find.
Rurouni Kenshin: The trope title is almost a word-for-word utterance by Old Man Okina after fully realizing that his most beloved disciple, Aoshi, has fallen entirely to the dark side—and is in league with Makoto Shishio, to boot. Specifically, he ask Kenshin to put an end to Aoshi's life. However, in a Crowning Moment of Heartwarming that also defies Broken Pedestal on Misao's part, Kenshin refuses to do it, he believes that Aoshi still possess a trace of goodness in his heart. A few hard knocks later, Aoshi realizes the errors of his way.

Comic Books

In Doctor Strange titles, it's not quite clear whether the Ancient One's other disciple Baron Mordo was this or a Deceptive Disciple. Maybe a little of both.
A slight variation in The Amazing Screw On Head: Emperor Zombie used to be Head's manservant until he turned to evil.
In Scott Pilgrim, Roxie was Ramona's protege before Ramona dumped her, prompting Roxie to team up with Ramona's other former boyfriends (the League of Evil Exes) for revenge.

Film

In Kung Fu Panda, Tai Lung was not only Shifu's first pupil, but also his adopted son. Tai Lung's Face Heel Turn was a direct result of their screwed up relationship.
- In the sequel, this can be said of Lord Shen and the Soothsayer, who raised him.
Named for a line in Star Wars Episode IV: A New Hope, about how Obi Wan describes Darth Vader.
- And mirrored in the prequel trilogy with Dooku, who was Yoda's pupil before his Face Heel Turn.
- And in the backstory, Qui-Gon Jinn had Xanatos, of the Jedi Apprentice novels, a downright nasty little psychopath and Corrupt Corporate Executive post-Face Heel Turn.
Betelgeuse in the movie Beetlejuice, worked for afterlife case worker Juno before he became a "freelance bio-exorcist."
Clu from Tron: Legacy. He took Flynn's original programming for him too far, and warped it. The comic Betrayal, however, indicates that he probably wasn't all that good to begin with, and Flynn was just too distracted by the pressures of his life in the analog world to notice until it was much too late.
Inverted in Bill And Ted's Bogus Journey, where it is Rufus's old teacher, De Nomolos, who turns to evil.
In X Men First Class, the mutants all train together as part a CIA team to fight Sebastian Shaw, but Angel Salvadore decides to join with Shaw, shortly after she is recruited by the program. At the end of the film Erik (Magneto) and Raven (Mystique), after defeating Shaw become the new Big Bads and turn against Charles Xavier, whose ideas about Mutant-Human relations differ from the ones they have, to form the Brotherhood.
The plot of The Hunted involves Tommy Lee Jones's character being asked to hunt down his former pupil, played by Benicio Del Toro, who has gone rogue.

Literature

Sauron, the principle antagonist of Tolkien's The Lord of the Rings trilogy, was a Maia in the First Age under the tutelage of the Vala Aule the Smith, and went by the name Mairon, before Melkor the Morgoth corrupted him to his cause and made him his lieutenant. When Morgoth was defeated, Sauron essentially became self-employed and sought to dominate Middle-Earth himself.
Morzan for Oromis in the Inheritance Cycle.
Nuihc for Chiun from The Destroyer.
A startling number of Luke Skywalker's Jedi students, in the Star Wars Expanded Universe, end up eventually falling to the Dark Side. He's able to bring some of them back. Brakiss intended to be a Deceptive Disciple, but Luke believed he could be turned; Brakiss started Becoming the Mask until Luke put him through a test which horrified him and caused him to flee. Luke always believed that Brakiss could be redeemed, and in The New Rebellion parts written from Brakiss's point of view suggest that he'd like that, but it's not to be.
Inverted in The Secrets of the Immortal Nicholas Flamel. William Shakespeare was a pupil of John Dee (the Big Bad of the series) but turned to good. Or at least, turned to an ally of the Flamels. How good this makes him is not that clear.
The Darklord Lorthas, former student of High Wizard Aemon in Boundary's Fall. He's still not entirely bad.

Live-Action TV

Played word-for-word straight by Jarrod in Power Rangers: Jungle Fury. His counterpart in Juken Sentai Gekiranger follows roughly the same path, but there is no single trigger that causes him to switch: he simply becomes dissatisfied with the progress he's making under the good guys.

Tabletop Games

Dungeons & Dragons
- Forgotten Realms setting. Elminster isn't really into it, but he taught a lot of wizards in his years, and the main requirement was the talent, not moral qualities - Mystra has priorities stated very clearly, and it's not like her own avatars never acted downright villainous. So... Sammaster - though after being "certified" by Mystra and before fully going nuts. The Shadowsil - became an evil archmage, worked in late Sammaster's little club, with virgin sacrifices and all that. Raerlin - has a dubious achievement of saying "Death has come for you at last, Old Mage" twice: once while alive, once as a lich. And so on, and on.
Horus in Warhammer 40,000 used to be the most trusted Primarch of the God-Emperor, who treated him like a son. Then Horus gets corrupted by Chaos and leads a rebellion that results in trillions dead, the Emperor in a coma, and the future of the Imperium in peril.

Video Games

According to the manual for the original Banjo-Kazooie, Gruntilda the witch was a student of Mumbo-Jumbo's until she turned evil... and to pay him back, she turned his head into a skull-shaped metal mask. It seems to have been Ret Conned, though, with Grunty's Revenge (which takes place in the past) has Mumbo Jumbo as an up-and-coming shaman without a pupil... and a face that's still skull-shaped.
Of the first Fire Emblem Garnef is an ex-pupil of Gotoh's. This part of the reason why Gotoh lacks faith in humanity.
The Legend of Zelda: The Minish Cap: Said by Ezlo about Vaati, who "became enchanted by the wickedness in the hearts of men."
Vargas in Final Fantasy VI, kills his father and master out of the belief that he favored Sabin.
In Tekken, Feng Wei, whose design is actually based on evil kung fu villains of the 70's wuxia flicks.
Uther Lightbringer and Arthas Menethil in Warcraft III were much like Obi-Wan and Anakin.
Isidora Morganach is taken under Percival Rackham's wing centuries before the events of Hogwarts Legacy. During the game, the player finds journals and viewable memories showing Isidora experimenting with dangerous magic and eventually turning against Percival and his order, The Keepers. Eventually, he is forced to lead The Keepers on a mission to stop her research.

Web Comics

Weregeek has it Played for Laughs by Dustin and Joel.
In Our Little Adventure, Thomas Stratus was one of Brian Souballo's old magic teachers back before Brian went all megalomaniacal.
Irregular Webcomic, as usual, picked on Star Wars: it's not like Obi-Wan failed worse than others, indeed.

Web Original

The Chaos Insurgency is a dangerous rival to the SCP Foundation, which according to one story, used to be part of the Foundation itself.

Western Animation

Fukushima from the Kim Possible episode "Exchange" was a student of the Yamanuchi Ninja School. He turned out to betray the school and helped Monkey Fist obtain a powerful weapon. It's not really explored why he did so, either he was already paid by Monkey Fist, or he only turned after Ron came to Japan. He could have felt insulted that Ron received honors that according to him should not be given to "outsiders". Or else he became jealous of Yori's shows of affection for Ron.
In Dungeons and Dragons, the Dungeon Master revealed that Venger was a former pupil. In the unaired series finale Venger is revealed to be Dungeon Master's son.
Transformers Animated. While never really elaborated on, Lockdown was once a pupil of Yoketron's. At some point, he abandoned the Cyber-Ninja dojo - only to return, sometime after Prowl joined, to make off with the dojo's protoforms and to kill Yoketron. Oilslick may also apply for this trope, but with the lack on information on his past, it's debatable.