Majin Tantei Nougami Neuro has Yako learning about the mysteries of humanity from a demon from hell, a sociopath killer, and the Yakuza. By the end of the manga, she takes all that she's learned from monsters and dedicates herself to peaceful conflict resolution regardless of country or creed.
Paptimus Scirocco of Zeta Gundam ends up serving as a mentor for several other characters. Naturally, his harem of newtype pilots comes to mind most easily, but Scirocco also mentors Jerid Mesa and Yazan Gable throughout the course of the series.
Ali Al-Saachez from Gundam 00 served as one to Kurdish Child Soldiers, going as far as to instruct them to become Self Made Orphans in the name of "god". One of those children, Soran Ibrahim, eventually became Setsuna F. Seiei, The Hero of the series.
Master Asia of G GundamtrainedDomon since childhood (before he became evil, though), and is revealed to be the Big Bad. He remains so for a good part of the show.
In the original Yu-Gi-Oh!, Gozaburo Kaiba was this to Seto, at least in his own mind. While he was exceptionally harsh and strict to young Seto - and even abusive at times - he was trying to mold Seto into the ruthless tycoon that he was, devoid of emotion and able to continue his military firm. Ironically, this was Gozaburo's undoing. He specifically told Seto that to succeed, he could not trust anyone, not even him, and while Seto grew to despise Gozaburo, that was a lesson he learned only too well, using it to take over KaibaCorp in a hostile takeover and once in charge proceed to destroy his adoptive father's life's work.
Divine from Yu-Gi-Oh! 5Ds. As leader of the Arcadia Movement, he told his students (such as Aki) that he was trying to help them control their Psychic Powers. Truthfully, he was purposely making them even more destructive, hoping to mold them into an army of psychic assassins.
The Big Bad of Basilisk, Tenzen Yakushiji, takes a boy in ( Koushirou Chikuma) and trains him as his apprentice and right hand, years before the events of the series start.
History's Strongest Disciple Kenichi introduces a whole organization of these in the manga after the arc covered in the anime called Yami. Unlike Kenichi's masters, Yami stresses the value of martial arts as killing methods. One member in particular Isshinsai Ogata aka the Saint Fist has taken an interest in Kenichi...
Orochimaru seems to collect apprentices, and most of them wind up either evil and/or slavishly devoted to him.
Tobi picked Sasuke up a while after he split from Orochimaru and did an even better job at this. The contrast between emo boy and the blank-faced teen who stabbed Naruto on his first appearance after the time skip was impressive, but the contrast between the shinobi who told his two minions 'no unnecessary killing' when they assaulted a whole fortress and the psycho who decided to off a village and threw away three loyal subordinates without blinking is also pretty wow.
Tobi had his own evil mentor: Madara Uchiha.
Shimura Danzou did a version of this, too, though he tended more towards Svengali territory. With the More Than Mind Control conditioning and what he did to Uchiha Itachi and all.
In Fate/Zero, Gilgamesh acts as this in a slightly odd way to Kotomine Kirei. It's not that Gil is evil, he just believes a person should do what brings them the most pleasure, which for Kirei turns out to be causing others pain (which Kirei hates). Gil then spends the series encouraging a reluctant Kirei to embrace his true self, which he eventually does to the rest of the cast's misfortune.
In Wee Tian Beng's The Celestial Zone, Xue Wu runs across one of these near the end of the series. Given that his insane competitiveness has been pushing him down the slippery slope, and that a villain recently gave him the If You Kill Me You Will Be Just Like Me speech right before he kebabed her, it doesn't end well.
Deathstroke of the DCU does this as a hobby. He once acted as an Evil Mentor to Terra, before he realized too late that she was even more evil than him. He also tried to turn Nightwing evil in the "Renegade" storyline, but Nightwing turned that around by acting as a Good Mentor to Slade's daughter Rose which led to her Heel-Face Turn. He also founded an Evil Counterpart to the Teen Titanstwice. Slade in general has a disturbing interest in corrupting the next generation of superbeings.
In Outsiders, while fighting Arsenal, he once mentioned that after he killed Roy he would take Lian and raise her to be an assassin.
Prometheus, also of the DCU, gives this a brief shot when he takes in a young sociopath named Chad Graham. He intends Chad to be his Robin, but the kid turns out to be a huge disappointment. Eventually he sets him on fire for his trouble.
Talon of the Fraternity of Raptors plays with this trope a bit. In War of Kings, he tells Starhawk the origin of his powers, omitting that the powers are actually for intergalactic espionage and assassination.
Granny Goodness was a mentor for several villains from Apokolips, but also trained Scott Free (aka Mister Miracle) and Big Barda, both of eventually becoming heroes.
The Scarecrow has tried this a couple of times, encouraging young victims of bullying toward bloody revenge. He's had various degrees of success—one of his "students" prevented him from stabbing Batman, but kept and used a can of his fear gas.
The Heart of Hush storyline revealed that twenty years ago Scarecrow played this to a young Tommy Elliot - better known as Hush.
Preacher: Jody, the man who killed Jesse's father, served as an Evil Mentor and substitute father figure to the young Jesse. Jesse hates Jody with a fiery passion, but he's man enough to admit that he wouldn't be the man he is without Jody's abusive lessons; he picked up what he knows about horses and engines from watching him, and when he puts every single round from his pistol in the ten-ring of a target and is asked who taught him to shoot, he answers "Sadistic fuckin' madman I used to know. Long story."
Hawkeye's mentor Trickshot, the man who taught him archery in the first place, was this at first. He trained young Clint to be a great marksman — so he could have an accomplice watching his back when he committed crimes. After being forced to shoot his own brother, Clint backed out of their arrangement, causing Trickshot to promise he'll live to regret that decision. When they meet again in the present day, Trickshot gradually becomes Hawkeye's ally if not necessarily his friend.
It's beginning to look as if Billy Butcher might be this to The Boys, although given the Crapsack World that the work is set in, it might be more of a Necessarily Evil Mentor.
Doctor Strange once had to learn dark magic from Kaluu, the rival of his original mentor known as the Ancient One, in order to combat the demon Shuma-Gorath. Deconstruction of the trope soon followed — Kaluu may have been ruthless or pragmatic, but he still did far more good than harm.
In Wanted, the entire Fraternity serve as this to Wesley, but Solomon Seltzer and the Fox in particular. Their goal is to make him a powerful and feared supervillain like his father, but in a subversion they do have his best interests at heart. Wesley's father is a more distant version, as he doesn't reveal himself until the end to complete his son's training.
The Taskmaster is a Marvel villain who does this professionally. Occasionally working for the Red Skull, he hires himself out to train mercenaries and super-villains; while some of his students have become successful super-villains - or even heroes, more or less - in their own right (examples include Crossbones and Cutthroat (also henchmen of the Skull), U.S. Agent, Hauptmann Deutschland, Diamondback, Spymaster, the original Spider-Woman, and Agent X) most of the time he just trains thugs to be low-rent henchmen and cannon fodder. (When working for the Red Skull, he can often cross the line to Treacherous Advisor at times, in one case sending the more disappointing ones to be "sparring partners" for his boss, which was a death sentence. On his own, he's more by-the-book, in case one hiring other super-villains to form formal academies, like the time Anaconda worked for him as a calisthenics instructor.)
In Farmer Weathersky, the boy's master teaches him magic but will keep him forever unless his father can find and recognize him, which he manages only with difficulty. Father Weathersky then tries to get the boy back with trickery.
In The Thief and His Master, the father only has to pay if he can't recognize his son, but the master uses magic to prevent him. And when the father succeeds, he tries to reclaim the boy.
Daniel to Danny in the Danny Phantom fanfic Daniel Masters. Slightly played with in that Daniel believes in Black and Grey Morality. He teaches Danny about some abilities, but—because he actually respects Danny's character—won't teach him the more... deadly skills until Danny asks.
Voldemort to Harry in the Harry Potter fanfic An Aunt's Love. Well, Bleys did say that Voldemort had to give him something twice, because of Voldemort's taking of his parents and his blood. This could be the way it happens.
Subverted in Harry's Savior, when Voldemort is actually turned back to the light.
Webwork: Jade's transformation back into a Shadowkhan already screwed up her moral compass, but she's further corrupted by the tutoring of Tarakudo and the Jorogumo Queen.
Films — Animated
Deconstructed in Megamind when the eponymous villain, bored half to death after defeating his archnemesis Metroman, decides to introduce a new adversary into his life. He trains unassuming cameraman Hal while disguised as Hal's "space dad". When he ambushes Hal later on after Hal becomes Titan and breaks the news that he was the one who trained him incognito, the roles are quickly reversed as Megamind is forced to stop Titan before he wrecks Metro City.
Films — Live-Action
Gordon Gekko from Wall Street. In this case it's the student who pushes for the mentor's advice. As soon as Bud Fox makes himself useful to Gekko's financial schemes, Gekko takes him under his wing and gives him the life he always wanted while teaching him to be driven solely by greed and use illegal means if necessary to ensure his profits.
Obviously, Anakin Skywalker and Chancellor Palpatine of Star Wars: "I can teach you things you need to know in order to save Padme. Oh, and you'll have to kill a bunch of kids for me too, but never mind that now..."
This actually happens so often in the Extended Universe (to Ulic Qel-Droma, Luke Skywalker, Jacen Solo...), it's practically a trope of its own.
The Karate Kid, part 3 had Mr. Silver, slowly training Daniel to become more and more vicious in his fighting tactics. This climaxes when Daniel punches another guy in the nose at a dance, which makes him realize what he has done.
Sebastian Shaw towards Erik in X-Men: First Class, who eventually took up Shaw's mutant supremacy ideology. A truly evil one at that, as Shaw killed his new student's mother in front of him to unleash the boy's powers and performed horrifying medical experiments on Erik.
John Milton in The Devil's Advocate. Curiously, he doesn't actively encourage Kevin to become an Amoral Attorney (while indirectly encouraging him by surrounding him with the perks of being one) because he wants Kevin to choose that path himself.
Some fans interpret Hannibal Lector as regarding himself as this to Clarice Starling in the film versions, but if so, it's entirely one-sided.
In Kill Bill: Volume 2, there's the Bride's sensei, Pai Mei. He's clearly a cruel man, and according to the film's mythology was responsible for the destruction of the Shaolin Temple because a Shaolin monk that he passed on the road didn't return the slight nod that he gave him. He's been known to snatch out the eyes of those who look on him with defiance and snap the backs and necks of those who give him sass like they were twigs. This eventually gets him killed when he snatches out Elle Driver's right eye for calling him a "miserable old fool," and she retaliates by poisoning his fish heads. He hates Caucasians, despises Americans and has nothing but contempt for women. Despite this, the Bride, an American, whitewoman, becomes his greatest student, the only one he teaches his Five Point Palm Exploding Heart Technique. She also leans to love him like a father, eventually avenging his death by snatching out Driver's left eye.
Training Day: Alonzo tries to groom Jake into becoming a Dirty Cop. When Alonzo fails to corrupt Jake, who's willing to stick to his principles, he ultimately sets him up to be killed by a bunch of gangsters.
American History X: The white supremacist leader Cameron Alexander manipulates whole scores of youngsters into the Neo-Nazi lifestyle by presenting himself as a wise mentor. Like a true evil mentor, he also has no problems selling out his followers when it suits him.
Justin DuMorne, Harry's guardian and first teacher in magic. Later in the series, the fallen angel Lasciel attempts to become an Evil Mentor by teaching Harry how to power his spells with literal hellfire. It doesn't work. If anything, Lasciel's image is "corrupted" by Harry, whose fundamental decency actually bleeds over into her.
Also Lea, his faeriegodmother who, while she never tries to corrupt Harry, is extremely dangerous and not overburdened with morals, beyond the usual faery sensibilities.
In Ghost Story Lea moves onto teaching Molly as well. And in a My God, What Have I Done? moment Harry calls himself one of these by bargaining with Mab for power while officially employed as Molly's role model.
Harry wonders if the reason he survived his battle with He Who Walks Behind was because the latter was trying to train Harry, not kill him.
Teresa Edgerton's The Castle of the Silver Wheel: the dwarf Brangwengwen plays this straight with Gwenlliant - Brangwengwen, a partially trained, elderly witch, knows the castle well enough to get into the Princess Diaspad's old rooms and thus to her old spellbooks (which feature Black Magic), and offers instruction in witchcraft to Gwenlliant (who otherwise has no teacher, and few people to talk to).
In In the Midnight Hour by Patti O'Shea, Ryne's mentor Anise turns to the dark side. Ryne doesn't know for sure during her years-long training period with Anise that she's secretly evil, but eventually figures out that she must have been bad all along. (You'd think Anise's kinky bedroom antics would have been a clue.) As the person closest to her, Ryne is assigned to be the one who takes Anise down, and she's afraid that she may turn to the dark side as well.
Another Evil Mentor's Book is the annotated Potions textbook in Harry Potter and the Half-Blood Prince: Harry has no idea what one of the spells written in the book does until he uses it on Draco Malfoy and severely injures him. However Malfoy was trying to cast an Unforgivable Curse on Harry at the time.
Falcone in the Warchild Series plays this role for Yuri. He attempts it with Jos and Cairo, too, but they don't jump at his call and both reject his teachings at once. Only Yuri follows Falcone's training and instruction without protest.
Joruus C'baoth in The Thrawn Trilogy calls Luke to him, and for a few days Luke follows him around trying to learn from him, but quickly comes to believe that C'baoth was insane and had possibly fallen to the Dark Side. C'baoth believes himself to be the ultimate authority, naturally above those who are not Jedi. This is cinched when Luke tries to leave with Mara Jade and they are attacked, and later when they find that he was working with Thrawn.
Interestingly played with in Outbound Flight, with Jorus C'baoth (the original) and his interest in the fourteen-year-old Anakin Skywalker. The original C'baoth had a superiority complex and beliefs much like his clone's, though slightly less obvious. Obi-Wan is uneasy about this. Anakin, in some of the most subtle this-kid-isn't-gonna-turn-out-right characterization in or out of the Expanded Universe, thinks that C'baoth is awesome. He solves things so quickly, and he doesn't take nonsense from anyone.
The Return of the Home Run Kid by Matt Christopher (sequel to The Kid Who Only Hit Homers) is essentially the G-rated version of this. The mentor is a former baseball player kicked out for betting against his own team, and he teaches the main character tricks like how to fake getting hit by a pitch. (It's not entirely clear how this is worse than cheating through magic in the previous book, but it's pretty clear that we're supposed to see this as a negative development.)
In the Second Age, Sauron taught Celebrimbor and his smiths the art of ring-craft, but never told them that the rings included binding magic (and could be controlled by the One Ring). He was also a full-on Evil Mentor to the last king of Númenor.
Morgoth was this to Fëanor in The Silmarillion, and was reasonably successful until he implied he wanted Fëanor's treasure.
Well, sort of. Morgoth claimed this, but in reality Feanor couldn't stand him, so he was more the mentor to everyone else. Which didn't help matters.
Viv Ivins in the Left Behind prequel books serves this role to Marilena Carpathia when she seeks to be pregnant and ends up becoming the mother to the future Antichrist Nicolae Carpathia until she is killed off.
In the Knight and Rogue Series the man whose name is not Jack Bannister is a varient of this for Fisk. The skills he was teaching were along the lines of conning and burlary, so it wasn't as though Fisk didn't know he was a bad guy, he just turned out to be a much greater Jerkass than Fisk had first thought. His last lesson to Fisk is that life sucks, and when Fisk finally recovers from the schooling, Jack tries to reinforce the lesson by having Michael tossed off a cliff.
In Ship Breaker, Nailer's insanedrug addictfather Richard Lopez, is the man who taught him how to swim, how to fight, and how to survive. This doesn't stop Nailer from recognising Richard for the dangerous loose cannon that he is, and while he is grateful to his father ultimately ends up killing him in the climax.
Light And Dark The Awakening Of The Mageknight: Played with. While Syndil is hinted to be this from his intro and does eventually Face-Heel Turn it's implied that he was driven by despair. He wanted the Mageknight to met his expectations. He did his best to develop his powers and those of the other knights until then.
In Sukhinov's Emerald City decalogy, The Wicked Witch of the East (Named Gingema in text) adopts Corina, a lttle Munchkin girl, and trains her in magic. Needless to say, Corina ends up massively scewed up (though not outright evil).
Mayor Wilkins represents an Evil Mentor / Parental Substitute to Faith, the series' renegade Slayer. While not directly causing her Start of Darkness, he genuinely cares for Faith very deeply. Perversely, Wilkins is the closest thing to a father that Faith has.
Also Gwendolyn Post, Faith's treacherous Watcher. Faith really can't catch a break...
The Master is one to the Anointed One.
Angel has Holland Manners, a senior attorney at Wolfram & Hart who considers Lindsey MacDonald as his protégé. He's actually quite similar to Wilkins from Angel's parent show, though he leans more toward the Ambition Is Evil side of things.
Averted with Zhaan in Farscape. She had to learn both offensive Psychic Powers and become willing to use them to beat the episode's Big Bad, both painful since she was a Technical Pacifist. Her teacher at the time was more of a Zen Survivor than evil though, but his lessons led to her using her abilities more assertively in later seasons.
Zhaan is a borderline example of this trope at best, because she had already used her powers to kill her lover in her backstory, who was planning to utilize the Peacekeepers to gain control of Delvia. This is in fact what led to her imprisonment on Moya in the first place. In other early episodes of Season 1, Zhaan herself makes clear that beneath her calm surface she is a potentially extremelyviolent individual. In fact Zhaan is asked to become the evil mentor of a band of renegade Delvians who want to learn for themselves how to use their powers to kill without going bonkers as a result.
Perhaps a much better example is the relationship between Crichton and Scorpius. A central arc over the course of the series consists of Scorpius attempted to convince John to utilize his wormhole knowledge as a weapon against the Scarrans. When Crichton finally does, he threatens to destroy the entire frelling universe unless both sides agree to peace.
In Season 2 of Heroes, Big Bad Adam Monroe takes under his wing resident Idiot Hero Peter Petrelli, successfully manipulating him into furthering his plans to release an apocalyptic virus upon the Earth.
In the same season, Sylar acts as a mentor to Maya, teaching her how to control her power without the help of her brother. And in late Season 3, Sylar himself meets up with his father, a Retired Monster who used to be just like him and who convinces him that he needs to stop hunting "small game" and go after real power. This eventually inspires Sylar to try to kill the President and take his place using Voluntary Shapeshifting.
Supernatural: The demons usually take up this role towards the heroes.
The Yellow-Eyed Demon, Azazel, fancied himself this and sort of managed it, in a ham-handed way, with the rest of his specials, but Sam has a personal grudge and Heroic Willpower and a big brother, so it never really worked. Ruby's the follow-up.
Ruby. She spends a season being mysterious and helpful before Dean dies, and then provides the bereaved Sam with emotional support, and encourages him to develop his Psychic Powers, which are of demonic origin, in order to avenge Dean. Even though he'd promised Dean he wouldn't use them. She gets him hooked on demon blood, a power booster that turns out to be highly addictive as well as revolting, and ultimately uses him to free The Devil. She was Lilith's inside woman all along. Even though he showed a lot of The Dark Side Will Make You Forget markers, he never got further than rather lukewarm Well-Intentioned Extremist...unless he actually exsanguinated that poor nurse Cindy, but that was a once-off. He trespasses rather badly against his family, but almost all of it is either under siren venom or in the throes of withdrawal. Sam is actually a really good guy, but no one including Sam really believes that even a little for much of season five.
Alastair to Dean. When Dean was in Hell he tortured Dean for decades until he broke completely, and then taught him the arts of torture so his student could apply his teachings on the new arrivals. Plays up the avuncular thing kind of the way Azazel used to. Thankfully no signs of Stockholm Syndrome, at least not that survived his resurrection.
Lucifer toyed with the role as well. These idiots will not leave Sam alone. On the other hand, Soulless Sam in six didn't need Samuel's influence to be a cold sonuvabitch.
Season six Crowley to Cas, a bit. Not that the latter isn't the more powerful, but the former leads him by the nose with the hope of knowledge he can use to end the war, and his partnership with the devil's replacement drags him down until the Moral Event Horizon makes a faint whooshing sound as it flies by. Somewhere around the time he started killing his friends so they couldn't stop him, maybe? And then he succeeds, and goes batshit insane.
In the eighth season episode "Freaks and Geeks", Victor Rogers takes in three orphaned teenagers and trains them to hunt the sort of monsters that killed their families. If that was all he did, it would be downright heartwarming by Supernatural's standards. Unfortunately, he's also the one who arranged their families' murders in the first place and had innocent people turned to play the part of the villains for the Roaring Rampage of Revenge.
Of a sort on Person of Interest. Kara and Snow act as sort-of mentors for Reese in the CIA, yet they are both series villains.
Michael admits that with hindsight, he can tell how unbalanced Larry always was. They worked together during a crazy time, so he seemed somewhat sane in comparison. It's also implied that Larry faking his death allowed Michael to spend more time with saner people, leading to him recognizing Larry for what he was.
On The Wire, Chris and Snoop, Marlo Stanfield's top enforcers, tutor Micheal in the ways of the game.
The mentor of Smallville's version of Green Arrow is a cold-blooded murderer and looks down on Ollie for becoming a superhero. In Season 5, Brainiac begins as one for Clark.
It's revealed late in Babylon 5's third season that The Shadows view themselves as essentially this. They spread chaos and war and corrupt people because they are Social Darwinists who want to encourage people towards their version of 'freedom' and help them evolve by cutting loose all fetters and encouraging and aiding those with ambition.
Morden: It's like knocking over an anthill. Every new generation gets stronger. The anthill gets redesigned, made better. Sheridan: So that's what the Shadows do. Come out every few thousand years, and kick over all the anthills, start wars, destroy entire races. Justin: A few get lost along the way, yes, and that's unfortunate. I don't think it was ever easy, but you can't let that get in the way of the dream.
Game of Thrones reimagines the character of Dagmer into one of these for Theon Grejoy. He feeds his pupil's ambition and feelings of insecurity, clinging on to feed off his success. When Theon intends to die in a blaze of glory alongside his men, Dagmer knocks him unconscious and sells him to their enemies for a ticket home. Ramsay Snow takes Theon but has Dagmer flayed alive instead when he captures the Ironborn, possibly for his treachery, but more likely because he just thought it'd be fun.
The Female Changeling from Star Trek: Deep Space Nine serves as this to Odo, with a strong dose of vamp. In "The Search", she teaches him about the history of the Changeling race and helps him hone his shapeshifting abilities. When Odo discovers that she and the other Founders lead the tyrannical Dominion, he leaves her. Later, during the occupation of Deep Space Nine, she teaches him more about his people and links with him.
Kamen Rider Ryuki had the Alternates, who believe that in order to save a thousand lives, they must kill at least one person and have been aiming to kill a central character for her ties to the Mirror World. They taught their student, a participant in the Rider War, the "kill one, save a thousand" motto. What they didn't bank on was that he'd take it to mean that if he killed them, he'd save a thousand.
Kamen Rider Double has Shroud, who gives her student his Transformation Trinket after he lost his parents to a Dopant attack. The evil kicks in when it's revealed that she was the one who gave the Dopant his Trinket as well. She also intended him to combine his powers with The Hero's partner to become a rider made of pure hatred. However, this is all subverted, since Shroud didn't intend for the Dopant to go on the loose and kill tons of people and that the student was just her Plan B in a Roaring Rampage of Revenge against the Big Bad who happens to be her husband. The student soon shows her that they won't rely on hatred to take him down.
Kamen Rider OOO plays this a little lightly with Ankh. He's more of a Jerkass who cares about absorbing Cell Medals instead of the well-being of others. Too bad the person he suckered into becoming OOO thinks otherwise and early on shows him what a bad idea it was for Ankh to give him the Driver, as he refuses to transform and even threatens to drop the Transformation Trinket in the ocean if he were to hurt a human. Like Shroud, this gets subverted, though in a different manner, since Ankh develops into a Jerk with a Heart of Gold who begins to question his nature as a Greeed.
Kamen Rider Fourze has four Evil Mentors. The first is the Scorpio Zodiarts, who hands Zodiarts Switches to people who are jealous, though he just gives the Switches and nothing else. Then comes the Libra Zodiarts who does more with the people he chooses to be Zodiarts, from having them call him "Master Libra", to disguising himself as someone else to see if the Zodiarts still got what it takes. The third Evil Mentor is the Leo Zodiarts, who takes some people in to teach them how to harness their combat pretty well. But by far, the better example of the Evil Mentor trope done in Fourze is the Virgo Zodiarts, who disguised himself as Tachibana and mentored Ryusei, leading him on the idea that he'll save his friend with the Aries Switch. In an odd twist, Tachibana acts like a classic mentor to Ryusei. If you guessed this is a subversion, then congratulations, you're catching on. The Virgo Zodiarts wanted to oppose the other Zodiarts without getting caught for it, hence the disguise, and when he revealed his true identity, he also revealed that he was Good All Along.
Kamen Rider Wizard managed to go four in a row with what is perhaps the evilest of mentors. See, where as the other three Evil Mentors had some connection with the Big Bad, Kamen Rider Wizard decided to cut the middleman and made The White Wizard and the Big Bad Wiseman one in the same, using not only Haruto (and later on three other Wizards) for his schemes, but also the entire Phantom race. It manages to double subvert this trope. His aim is to save his daughter. A noble goal, right? How does he achieve it? Well, murdering dozens of people and having monsters taking over them, some of whom have killed people in their wake as well, indirectly making a serial killer very powerful, kidnapping a kid and an expecting father and brainwashing them into becoming Wizards, and ultimately planning to repeat the first step seems like a pretty huge leap off the slippery slope just to save someone.
Parodied in Dilbert when Wally mentors Asok—the presentation is comparable to serious examples of this trope, but the actual content is training in how to avoid having to work while still getting paid. Asok may or may not be becoming more like Wally, but if he is, it's a slow process.
Roy Cohn to Joe Pitt in Angels In America. Despite being a hideously loathsome man, he has a genuine affection for Joe, and offers him much in the way of (what he thinks of as) advice, which later leads to problems when Joe refuses to believe his "questionable but good-at-heart" mentor could do the kind of horrible things Roy Cohn really did.
Kreia states fairly early that she used to be a Sith, and tries to impart upon the main character the lessons of self-reliance, how doing good rarely solves anything, manipulating others to do your bidding, and a whole lot of other lessons that run contrary to the Jedi code. She is also the Big Bad, but unlike a Treacherous Advisor she never uses her mentor position to backstab the protagonist, as she actually cares for The Exile.
Certain elements of the story allow the player character to play this role for the Handmaiden, the Disciple, Bao-dur, Mira, and Atton if you try to turn them to the Dark Side.
Riku had this as a recurring problem. The power he learned from Maleficent and Ansem was too useful not to use, but gave him serious self-image issues.
Terra and Master Xehanort in Birth by Sleep. After Terra's brief use of dark power costs him his Mark of Mastery exam, Xehanort takes advantage of Terra's strained relationship with The Obi-Wan (Eraqus) and offers him the validation he craves. While Riku later proves that darkness can be safely channeled, Xehanort's "comfort" is entirely self-serving. It's like Anakin and Palpatine compressed into one story.
Goutetsu in Street Fighter, who taught a form of martial arts that used murderous intent in every move. Two of his students were Gouken and Akuma; Gouken purged the murderous aspects of the martial art and went on to teach this form to his students Ryu and Ken, while Akuma used the form as it was intended. Akuma used it to kill Goutetsu (who died happy, knowing his legacy would carry on through his student) and later tried to pass it on to Ryu.
The manga adaptation of Street Fighter Alpha by Masahiko Nakahira deviated from this back-story by making Goutetsu the one who purged the Satsui no Hadou from the art itself.
Houzuki from Sharin No Kuni is not only an Evil Mentor, but also the Big Bad, and doesn't make any attempt to disguise what he is - his Establishing Character Moment is shooting one of the trainees under his tutelage for being late to an appointment and Kenichi, the protagonist and his protege for the last seven years, is under no illusions as to what sort of person he is.
The Baron in Amnesia: The Dark Descent is revealed to be this for Daniel towards the very end - though if the player didn't suspect this long beforehand, they might be carrying the Idiot Ball.
A book in Oblivion talks about how Nocturnal's cowl was stolen. A young thief tries to steal from a master thief, who summarily catches her but lets her be his protege. Eventually they plan a heist to steal the cowl of Nocturnal. The young thief is to wait in a nearby tree, wait for Nocturnal to remove her cowl, then let the elder make a distraction. When the distraction never comes, the young thief attempts to steal the cowl without help. After she is caught, she looks over to realize that Nocturnal's cowl is gone. When asked who she is, she says, "I'm the distraction."
In the Yu-Gi-Oh! video game World Championship 2010: Reverse of Arcadia, the Player Character is in much the same situation that Aki is in, with Divine as his/her mentor. (And plays a big part in defeating him and the Dark Signers; the storyline of the game is similar to the that of the Anime, but has your character substituting for many key characters in important roles.)
Skull Girls: Valentine has a more adversarial version of this with Painwheel, at least in the former's story mode. She "created" Painwheel in the first place, and wants to see the girl-turned-monster become strong enough to kill the Skullgirl. Even if Valentine has to become a Skullgirl to do so. This relationship is best summed up in her dialogue whenever Painwheel beats her: "Atta girl".
In the video game tie-in to The Amazing Spider-Man 2, Kraven the Hunter acts as this to Spider-Man. He teaches Spidey his hunting techniques and how to apply them to crimefighting, but only as a pretense to study the wall-crawler and figure out how best to kill him.
Bernkastel from Umineko no Naku Koro ni serves as a meta example to the player as much of her words are actually useful ways to think of the mystery. She's essentially a MUCH more malicious version of Kreia above. Also a rare non corruptor example
Miranda West in The Wotch may qualify for this, given that she's keeping some very big secret from protagonist Anne and that she's willing to destroy the lives of a few people in order to make sure nobody interferes.
Homestuck: Doc Scratch, the host of "a very evil man", considers himself to be a mentor (or "mentor's mentor's mentor") to several characters, some who stay good and some he turned evil.
Aggressive, greedy Meenah, a teenage alternate-universe version of The Troll Empress, wants to be a friend and mentor to her Lighter and Softer tween clone/ancestor/descendant Feferi but can't due to a genetic compulsion to kill her over the throne (never mind that by the time they meet they're both dead and their planet/galactic empire/universe no longer exists) and declares "maybe some day ill find an heiress who my genes dont instinctively make me wanna murder on sight then i can teach her the badass ways of being a boss n shit!"
In the "present", human girl Jane Crocker is heir to the Betty Crocker baked goods empire, unaware that "Betty" is the Troll Empress.
Though the "evil" label threatens a whole can of worms, Vriska mentored John over the course of Act 5-2 in all her Morally Ambiguous Base Breaker glory, and like many other examples started to achieve some measure of redemption via this association, while still committing some of her most heinous crimes ( murdering Tavros, facilitating Jack's ascension, and almost allowing Jack to slaughter the Trolls) in this time span.
Raven: It's dark magic! You've been teaching me dark magic! Malchior: Is it dark, or is it simply misunderstood... like you?
Ironically she ends up beating him with the same dark magic he taught her.
Slade and Brother Blood, too, at various points. Blood does this for a living.
Avatar: The Last Airbender: Katara meets Hama, a waterbender from their South Pole tribe, who uses her waterbending to take control of other peoples' bodies by "bending" the water inside their blood, and uses this power on innocent people in the Fire Nation town where she lives in revenge for her tribe being imprisoned by the Fire Nation. Katara is horrified and refuses to learn, but in the end must resort to using it on the Evil Mentor in order to save Sokka and Aang. Before being sent away, Hama "congratulates" her for using it and laughs. Katara was understandably upset. Ironically she used it on a Fire Nation soldier whom she thought killed her mother, but she's likely angry at Hama for teaching her bloodbending in the first place.
In Book 2 of The Legend of Korra, Korra herself is instructed briefly by her uncle Unalaq, that gives her some spiritual training and, like with Hama and Katara, a new waterbending technique, that infuses water with light. This bites him severely in the ass at the season finale when Korra uses this very same technique to purify Unavaatu.
In the final season The Secret Saturdays, V.V. Argost offers to teach Zak how to control his Kur powers. Being Argost, he admits to Zak right at the start that he intends to kill him in the end and take his Kur Powers for himself. And Zak still accepts...
In Transformers Prime (and its prequel book, Transformers: Exodus), Megatron was this to Optimus Prime back when they were Megatronus and Orion Pax. Megatron eagerly resumes this role once Optimus loses his memory of having become a Prime. Played with a bit in that it wasn't really until Orion Pax really started to become The Hero that Megatronus started to become jealous enough to start becoming the Big Bad; the situation is almost an inverse of A Pupil of Mine Until He Turned to Evil.