I didn't teach him the most important thing I had to tell him; there are some things you have to pass on. The trick is to know which one. All I taught him were skills. And now I have to stop him from using it to destroy us all.A mentor once had a student, who used to be a good person, and perhaps the mentor's most promising and skilled student ever. That student had a flaw; a seed of villainy that needed a trigger. Usually, it was arrogance about his/her skills, or impatience at not being taught the final skills that the mentor had yet to teach. Then the student became a villain. Sometimes the mentor is The Protagonist and will have to take the student down in the story, resulting in an Older Hero vs. Younger Villain scenario. More often, the protagonist is another student the mentor takes on. If that's the case then the other student is the Mentor's New Hope and the former student is a Hero's Evil Predecessor. 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. Contrast Treacherous Advisor, which is when the mentor betrays the student. 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.
—Peter Stillman, Metal Gear Solid 2: Sons of Liberty
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Anime and Manga
- In Soul Eater, Asura was Shinigami's apprentice and as it turns out, son until he went insane and threatened to destroy the world, and had to be sealed away.
- Orochimaru was the Third Hokage's student, considered the village's best ninja when he was in Konoha and 'good' enough to cry when Dan died. Then got fed up with things and left for his various evil deeds. 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.)
- There's also Sasuke, Kakashi's former student who abandoned the Leaf Village to train with Orochimaru, and then goes on a Roaring Rampage of Revenge once that's done. At one point he even allies himself with Akatsuki. Even after his talk with Hashirama, he still planned on killing all the Kages and taking over the Leaf Village after the war was over, requiring Naruto to beat even more sense into him.
- 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.
- Danzo could also be considered sort of an example, as he used to be a student of the 2nd Hokage. While not strictly evil as he thinks his actions are what's best for the village, he's pretty much constantly doing morally questionable things and getting involved in Dirty Business.
- Then there's Obito, who became Tobi, the leader of Akatsuki and The Dragon to Madara Uchiha. Like Orochimaru and Nagato, he played a big role in the death of his former teacher. However, unlike most examples of this trope, his teacher never even knew that he had turned evil, or that he was even alive.
- 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.
- Yu-Gi-Oh! GX: After his Freak Out, Kaiser Ryo Marufuji defeated his old mentor and stole one of those dangerous forbidden decks, but the sad part is Sameshima incorrectly thought he was in there somewhere.
- Demon City Shinjuku. Levih Rah was once the student and ally of his master Agni Lai, who taught him the art of Nempo. Unfortunately he was envious of the power of other men and ended up selling his soul to the demons of Hell in return for vast power.
- 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.
- La Quête de l'Oiseau du Temps has Bulrog; a former student of the knight Bragon who turned mercenary and eventually ends up seeking vengeance on his master.
- A slight variation in The Amazing Screw-On Head: Emperor Zombie used to be Head's manservant until he turned to evil.
- Inverted in Death Vigil by Wulf, who was Sam's mentor in the Vigil. Wulf was a loyal and respected member of the Vigil for centuries, but then his gifted surrogate daughter was killed in an accident and Bernie was unable to do anything for her. Although we never hear her reasoning from Bernie's perspective, Wulf claims she refused him, and didn't take it well.
- Jason Todd, the second Robin who died and came back later wearing a different mask, although whether he wants to (lethally) simply stop all crime from Gotham or murder the whole Batfamily depends on the writer.
Films — Animated
- 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 Kung Fu Panda 2, this can be said of Lord Shen and the Soothsayer, who raised him.
- In My Little Pony: Equestria Girls, Sunset Shimmer is a former pupil of Princess Celestia (and Twilight Sparkle's predecessor in that role). When Celestia's tutelage did not turn out to be the easy path, she "turned cruel, dishonest and rebellious," abandoning her studies and pursuing power in more unsavory ways. Unlike most she is redeemed, though she has yet to return to face Celestia.
Films — Live-Action
- In Star Wars, Anakin was a good man, if arrogant and quick to anger, until he was seduced by the dark side. Much of Luke's arc is if he will go the same path.
Luke Skywalker: How did my father die?Obi-Wan Kenobi: A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi Knights. He betrayed and murdered your father.
- The Trope Namer is a line in Star Wars Episode IV: A New Hope, about how Obi-Wan describes Darth Vader.
- Mirrored in the prequel trilogy with Dooku, who was Yoda's pupil before his Face–Heel Turn.
- And now gone full circle in The Force Awakens with Kylo Ren, one of Luke's students (and his nephew by way of Han and Leia, whose real name is Ben Solo), going to the Dark Side (thanks to his obsession with his grandfather, the Trope Namer), slaughtering all the other students, and serving the First Order in an attempt to defeat the New Republic. Ren then kills Han.
- 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 (even if for gym), De Nomolos, who turns to evil.
"I worked within the system until I could stand it no longer.''
- 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 the Hellfire Club shortly after she is recruited by the program. At the end of the film, Magneto and Mystique, after defeating Shaw, form the Brotherhood and become the new Big Bads, turning against Charles Xavier, whose ideas about mutant-human relations differ from the ones they have.
- 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.
- Tommy Gunn from Rocky V, who starts out as Rocky's loyal protege, but becomes bitter when the public sees him as Rocky's puppet, and joins with a sleazy promoter instead.
- A reversed version in The Mighty Ducks between Gordon Bombay and Jack Reilly.
- In Ant-Man, Hank Pym had mentored Darren Cross but Cross grew resentful that Hank was unwilling to share his research on the Pym Particles and is driven to get the same results from the Pym Particles to prove he was better than Hank, leading him to supply HYDRA with weapons.
- 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 Aulë 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.
- A 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.
- Played word-for-word straight by Jarrod in Power Rangers Jungle Fury. Rio, 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.
- Gekiranger also had the three Kenma, precursor villains who turned on the founder of Beast Fist after their leader learned he was the *second* choice for successor. This led to the Kensei sealing them away until Rio and Mele let them out.
- Ian Rotten helped Chris Hero, giving him the most recognition in his career till then in IWA Mid-South, as well as giving him some instruction to improve in wrestling. Hero would go on to turn on Rotten and all things IWA Mid-South just because of a loss to Arik Cannon.
- Tommy Dreamer found this after taking Colin Delaney "under his wing". This had no resolution in WWECW, in Delaney was released but Dreamer has since had to deal with his misdeeds elsewhere, such as the International Wrestling Cartel.
- Mr. 450 turned on his teacher, Invader #3 in the World Wrestling League after Invader decided attacking Sensacional Carlitos after their Navida Corporativa match, after Carlitos had displayed good sportsmanship, was a step too far.
- 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 — it's Mystra's priorities and not that her own avatars never were 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, virgin sacrifices and all that. Raerlin — the guy 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.
- In HeroQuest, Big Bad Morcar/Zargon was a student of Mentor who got fed up with not being taught magic fast enough, read texts forbidden to him in secret, gained great power, and ran off to become the Lord of Chaos.
- 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.
- In the backstory of the first Fire Emblem game, Gharnef was once a pupil of Gotoh, the White Sage. However, when Gotoh passed the legendary magic tome Aura on to his other pupil Miloah because Gharnef, while a good man, lacked compassion, he was overcome by jealousy and stole the Darksphere, a cursed artifact that twisted him into the cruel Evil Sorcerer seen in the game proper.
- 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.
- Demolitions expert Peter Stillman accidentally gives rise to Metal Gear Solid 2: Sons of Liberty's Mad Bomber, Fatman (real name unknown), a wannabe public enemy who has turned his knowledge toward terrorist bombings. Like any good Star Wars yarn, Stillman is cut down by one of Fatman's booby traps, leaving the callow, blond hero to finish the job.
Stillman: Microwave. With a 7-foot range. It's not a technique I taught him. Neither was that multi-bomb booby trap. Looks like he's far surpassed me as far as explosives technique goes.
- Uther Lightbringer and Arthas Menethil in Warcraft III were much like Obi-Wan and Anakin.
- Legend of Legaia
- Songi is this to Master Zopu and a Rival Turned Evil to Gala. Zopu and Maya both state that Songi was a good child, but his pride and anger at always being second best to Gala caused his decent unto evil.
- Kazan's former pupil was the swordsman Rauss. The latter was gradually corrupted by the Ultimate Fang into a Blood Knight, and his desire to fight his former master led him to a Face–Heel Turn ending with the master killing his former pupil.
- The Naughty Sorceress of Kingdom of Loathing can be found in Fernswarthy's ethics class. He wasn't that nice a guy, but she wasn't that good a student either.
- In War Of Omens, Listrata dedicates her life to the task of avenging her parents death. Step 1 of that plan is conquering an island chain (the Phenetik islands) as a thesis. However, her mentor, Ashkar views Listrata as a thug for her treatment of the Phenetikians during her conquest.
- Kingdom Hearts: The first six members of Organisation XIII were pupils to Ansem the Wise, the king of Radiant Gardens who helped him in his research into the Heart. Eventually, they turned on him and turned themselves into Heartless. Only not really. Only Xehanort and Braig apply. The other Apprentices were forcibly turned into Heartless.
- Weregeek has it Played for Laughs by Dustin and Joel, which is basically them doing the confrontation talk between Kenobi and Vader in A New Hope, but in terms of being a Dungeon Master.
- In Our Little Adventure, Thomas Stratus was one of Brian Souballo's old magic teachers back before Brian went all megalomaniacal.
- In Avatar: The Last Airbender, Admiral Zhao served as this to the firebending master Jeong Jeong.
- 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 & 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.
- Ewoks. The second season episode ''The First Apprentice" features Zarrak, who was master Logray's first apprentice before he turned to the dark side.
- Kuvira of The Legend of Korra was once a promising apprentice of Suyin Bei Fong that she treated as one of her own children and was Captain of the guard of Zaofu. After the fall of the Earth Kingdom due to the events of Book 3, the two had a falling out over whether or not Zaofu should militarily pacify the now-fractured Earth Kingdom. Kuvira eventually took Suyin's elder son, her engineer, the security forces as well as Zaofu's richest residents to form what eventually became the Earth Empire.
- Inverted in Men In Black The Series where Big Bad Alpha was Kay's mentor until he came across a powerful and illegal alien device that allowed him to assimilate alien body parts.