Quite funnily, Kid Paddle's protagonist fears this will happen to him under the same circumstances, suspecting his father to want it. He fears he would be turned into a Glurge Addict, a thought that apalls him.
This is probably Older Than Feudalism, as an apocryph legend says that Lucifer was originally the most beautiful of the angels of God and was called "son of the morning", untill he began to want godlike power / supremacy over humans, fell prey to Pride, was banished out of Heaven and became Satan.
Terry Dean in A Fraction of the Whole is as a child an extremely promising sportsman, but has a slightly psychotic obsession with fairness. His brother Martin uses this to manipulate Terry into attacking Martin's bullies, which results in Terry being stabbed in the leg and being unable to play sports. Frustrated, he turns to petty crimes which gradually turn worse, and he ends up making headlines as he goes on a killing spree against famous sportsmen in any way guilty of cheating. He then seemingly dies in a fire, but turns up again decades later, near the end of the book, as the morbidly obese kingpin of a crime syndicate.
In the House Of The Night serie, Stevie Rae became a more aggressive, impulsive, and slightly misanthropic undead after being The Cutie for most of the series. Unlike most of the initially heroic examples, it however contributed to improve her character, after she healed from her new aggressive nature, and to make her an independent, always heroic and more self-controlled person.
Probably an unintentionnal example, but Erik Night went all the way from a confident, open-minded and self-sacrificing young man who is asked out by his slightly possesive ex-girlfriend to a self-centred, I-mostly-don't-care-about-the-good-versus-evil-thing, self-doubting and possesive boyfriend who doesn't want to try to understand his newer girlfriend and shows off his newest girlfriend, the Evil Twin of the first girlfriend who Took a Level in Kindness and treats the heroine well, understanding the problems his being possesive can cause. This is treated as an example of Jerkass All Along.
It is difficult to say whether Neferet's warm, easy to trust and maternal nature has changed since she became a priestess, or if it was a mask since more long ago. She is revealed in tome 2 as a ruthless and manipulative figure towards Aphrodite and Zoey. On a similar way, a Big Bad's reputation underwent this, to the great surprise of Zoey and her friends. It is never revealed whether he actually was a case of this.
This is downplayed with the antagonist in Mansfield Park. Mary Crawford starts in Jane Austen's Mansfield Park as a somewhat vain gold-digger, who is nonetheless a loyal friend and had some kind of elegance about her which prompted much Draco in Leather Pants, while the heroine Fanny Price starts the book as a passive and judgemental extreme doormat who lacks self-condidence, prompting much Ron the Death Eater. In the end, Mary Crawford shows what remains of her initial True Colors, and borders Stupid Evil, having become geniuely interested in Edmund and a False Friend to Fanny. This is, as often, mirrored by a Hourglass Plot, as the initially defenseless Fanny shows more confident and useful traits, and gets what Mary Crawford wanted.
In Star Wars's Expanded Universe, it is downplayed with Jacen Solo, a compasive and self-doubting young Jedi, who seemed to be very generous to his family was turned into a Sith because he thought he could use the power of Siths without corruption. When his extremism got the better of him, he became a ruthless and inflexible Sith Lord, who killed his aunt, and only began to regain his previous characteristics when he feared for his daughter and questionned his lack of empathy, self-centred behavior, and uncontrolled outbursts of anger.
Live Action Movies:
Peter Parker, the Adorkable, awkward and relatively devoted suitor and hero from the first two Spider-Man movies briefly becomes a confident and manipulative egotist after fusing with an evil symbiote in Spider Man 3.
Live Action TV
In a Soap Opera, it can mostly happen when a character succumbs to his alternate personality.
Coincidentally, it happens to anyone who has ever been turned in the whole history of the series, except Spike, who kept his capacity of altruism even though he behaved terribly in many occasions.
It happened to Duncan. after the dark Quickenining, and to the warlord Darius after a light Quickening in the series Highlander.
In Legend of the Seeker, Cara is revealed to have been a vulnerable little girl who could easily be manipulated, was very emotional and couldn't bear the thought of leting a little fish suffer. Her vulnerability to manipulation and her impulsive nature lead to her becoming a collectedNoble Demon working as a Torture Technician. Later on, this is averted, as she regained part of her old personality and became an heroine while remaining extreme and stoic.
In Merlin, Morgana was at first a confident, brave, and always compasive young woman, who didn't make a difference between magicians and non-magicians. After she discovered she was born with a magical nature and would be hunted by Uther and shunned in Camelot, she became insecure and paranoid. Her feelings of isolation and lack of self-worth made her more self-focused. She slowly began seeing everyone in Camelot as an enemy and reinterpreting her relationghips with them. After a bertrayal, she began to feel justified and considered all non-magic users in Camelot as worthless and crual enemies who deserved to die. She then used her insight in their inner life to trap them, manipulate their minds, and ruin their romantic plotlines.
Arthur: What happened to you, Morgana ? I thought we were friends.
Morgana: As did I.
Though an unwilling example, the Sir Lancelot from Merlin first appeared as a generous and loyal knight with an unconspicuous but significant lack of self-confidence, but being brought back as a shade by Morgana and being forced to integrate the personality she wanted untill he would return in the Spirit World made him behave crually, act as a traitor, and finally, display overconfident Handsome Lech and Manipulative Bastard behaviors towards his New Old Flame Guinevere.
Speaking of which, one of the points of the show is to introduce us to an impulsive and clumsy teenage servant, a self-conscious Shrinking Violet, the aforementionned straightforward lady who is a protagonist and a slightly immature prince, and to turn them into a wise trickster mentor, a High Queen with a hint of Pride, a much more complex and doubtful antagonist, and a Good King.
The young Clark Kent from Smallville became a shadow of himself whenever he was exposed to red kryptonnite, or any product that could alter his personality. It was sometimes hinted by Martha, one of his mothers, to be a case of True Self revealed, which would mean that Clark mainly spent the first four season of Smallville willing to blast Lex against a wall while cheating on the poor Lana, all while quite conspicuously ruining his reputation and throwing money by the windows.
The same happened to Conner Kent, briefly.
Just as briefly, it happened to Lana Lang who went from a shy and polite girl to an uncivil attention seeker for an episode after smelling strange spores.
This seems to have happened to Tess Mercer, who was seen in flashback as an innocent, soft, optimistic and vulnerable young woman when she met Oliver. Cue to her being a Well-Intentioned Extremist, who can fight with almost everyone without superpowers, candidate to being the Big Bad and fully believing that Humans Are Bastards.
In Macbeth, the eponymous character turns into The Caligula, after being a loyal general who proved himself to be heroic and have qualms about his wife's plan to take power. He slowly loses all moral scruples, while his wife, who pushed him to ruthlessly betray his king, feels deep remorseand is Driven to Suicide.
This slowly happens to the prince in one of the sequels of Prince of Persia, when the side-effects of the sands of time corrupt him.
In The Sims, this can happen if a Sim drinks the yellow potion of the potion-maker machine.
King Leoric of the Diablo series went from a righteous and noble king to a bloody-handed madman and eventual undead abomination by the time that Diablo and his Evil Chancellor Lazarus got through with him.
It is discussed in Kid Paddle. The child protagonist imagines it can happen by surgery, turning his brain around. He becomes terrified at the idea that his sister and his father would do this to him. It is a nice metaphor of his impression that they do not like him, and that there is not a thing they accept in him. They actually care more, but it is made clear that he and them have almost nothing in common.
GIFT can consist of this, as it is wittily explained in this article. The reader is to jusge whether the purposed equation performs well or not in its prediction purpose.
The popular conception translated in In Vino Veritas suppose it happens when someone completely represses a great part of their natural personality and drinks too much. That, or alcohol just makes you act goofy for no reason.
This trope is frequently invoked in Real Life when cases involving extreme social pressure to avoid some behaviors and opinions and some ill-considered secret or denied traits being suddenly uncovered. Someone tries to hide one's opinions, one's religious beliefs or one's sexual orientation, covering up what could indicate its existence and vehemently denying it (sometimes even to oneself), untill it suddenly appears and generates either Internalized Categorism (and actual changes) or demonization from their peers, giving birth to tropes such as You Are What You Hate.
When the inability to hide something stems from a trauma, the deduction is often that the trauma caused the change. This is how a Sociopath, a Narcissist, or a Con Artist can be spoted in periods of stress after lapses in judgement, lack of planing, and sudden and ill-advised outbursts of rage... or just considered to be having a hard time. Even unable to manipulate, the Manipulative Bastard can still outsmart everyone in the end.
Real Life: Psychology
The concept of Shadow Archetype istelf is an interesting take on what someone rejects as foreign despite the fact that it is very much a part of one's possibilities and could manifest in one's behavior despite denial. This seems to cause the belief that people suffering DID have an alternate personality which is the contrary of the principal one.
It has been supported by the MBTI personality test's theoricians that, whenever an unstable person with a given MBTI type is confronted to extreme stress, she adopts behaviors of the "shadow" type, giving in to her own "shadow", and therefore using their cognitive function in reverse order of importance if not even using the cognitive functions they are not supposed to use naturally.
Studies lead by enlighten amateurs on message boards suppose that this stereotypically tends to involve someone with a type that is supposed to demand a lot of conscious effort to uphold his way of life lapsing into a more instinctive behavior which betrays one's values. The most commonly reported example is a stereotypical INFP (Introverted iNtuitive Feeling Perceiving) personality type (refered to as "an INFP") becoming an "evil" ESTJ (Extroverted Sensing Thinking Judging). Other common examples include a stereotypical ISFP (Introverted Sensing Feeling Perceiving) becoming an "evil" ENTJ (Extroverted iNtuitive Thinking Judging), and a stereotypical INFJ (Introverted iNtuitive Feeling Judging) becoming an "evil" ESTP (Extroverted Sensing Thinking Perceiving), which sounds very puzzling, at least to people who haven't been exposed to the MBTI stereotypes. Rather strange discussions on the otherwise perfectly understandable PersonalityCafe site center on this, enlighting both the irrational reaction provoked by people who identify with a shadow type of the type one identifies with, and the deep emotional reaction this trope provokes. This is losing oneself per excellence, after all.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.