Pictured above: Ciel Phantomhive. He's a cynical, callous, and very driven 12-year-old (or 13, depending on how far you are in the series), is a Chess Master and Wise Beyond His Years. He has no qualms about getting involved in dangerous missions and will command his badass butler Sebastian to kill someone without a second thought. He has also been shown slapping and insulting individuals for being too comfortable with touching or speaking to him. The only individual he appears to genuinely care for is his fiance, Elizabeth Middleford, although he does seem to view her as a bit of a distraction and a nuisance, at times. In the anime, he appears downright cruel at times, although in the manga, he is simply just icy and elitist. It's eventually revealed that he was a happy, loving, carefree child before his tenth birthday. Then his parents were murdered, the mansion set on fire, and he was kidnapped. He was locked in a cage with other children his age and eventually was to be a child sacrifice until he accidentally called upon a demon in his desperation to live.
Alois Trancy in the second season. He was shown to be cute and innocent as a child but then his foster family died in front of him, from unknown supernatural means, including his adoptive brother and only friend, and then he was horrifyingly raped and abused by his master, Lord Trancy, alongside other boys. A few years later he's shown to be creepily Ax Crazy.
Jotaro Kujo. First we see him in past as an little boy playing with mom and as an model student, and on the next page he's in a cell, terrorizing fellow prisoners and telling his mother to shut up.
Subverted later on in the manga when Alessi uses his stand to de-age Jotaro to about seven years, and Jotaro was still capable of pounding Alessi pretty seriously, and still had his not-accepting-any-nonsense mentality which, given the way Alessi's stand works (its victims don't remember anything from the years they lose), Jotaro must have had even then.
Diego Brando was shown to be a really nice kid in his childhood. That is until his mother died from being overworked by the unfair farm owner they were working for. That combined with the knowledge that his father had abandoned him drove him to become ruthlessly ambitious to prove that he's above people like his father by any means necessary.
Subverted with his original universe counterpart Dio Brando though, who despite having a similar abused childhood is noted by Speedwagon to be someone who's completely evil by nature.
Roberto, The Dragon from Monster, is retrospectively revealed to have been a sweet kid (gave up his cup of hot chocolate to give it to a sick friend, liked to collect bugs but would always let them go because he couldn't bring himself to kill them). Then the Orphanage of Fear really got to him - and as for the later character he shows in the story proper, let's just say "wouldn't hurt a fly" is not the phrase that leaps to mind.
Johan Liebert, the series' Big Bad, is about a deconstruction of this trope.
Peter Capek is another example. Once an ordinary kid, now a Mind Rape-inflicting fiend.
Even played with with Peter Capek, as his semi-Forgotten Childhood Friend, who grew up to be a normal dentist, attempts to kill him for what he's become at one point. His friend is shot dead before he can get a shot off, and seeing his now-remembered friend's death effects Peter for the rest of his life (a few more days).
Michio Yuki from Osamu Tezuka's MW is a sweet-natured, rather shy kid till the age of nine when he gets, in short order, taken hostage, molested, and exposed to potentially lethal neurotoxins that fuck up his mind terribly. After that, he goes "a bit" off the rails.
Osamu Tezuka loved this trope. Most incarnations of Rock have a similar story. Perhaps the most heartbreaking example is Adolf Kaufman, one of the three title characters of Adolf. As a child he befriends a young jewish boy and resists his Nazi father's hateful ways, but he's soon shipped off to a Nazi training school and it all goes downhill from there...
In Full Metal Panic, one side story focusing on Sousuke and Kalinin's backstory showed that Sousuke was this. During his childhood, when he was rescued by Kalinin and they spent time together, Kalinin was shown to love Sousuke and grow so attached to "that sweet and gentle boy" to the point where he really wanted to adopt him as his own. Then they got separated, and the next time they meet, they met as enemies, with Sousuke becoming the Nietzsche WannabeStoic that we know today.
Naruto has done this repeatedly with major characters.
As a child, Nagato looked like he could be The Messiah. Then...things hit the fan, he renamed himself "Pain" and decided it was better to be a Dark Messiah.
Orochimaru was one of the Legendary Sannin, and he showed great promise as the next Hokage. He was a kind and helpful orphan, but then he got the idea that he could either bring his dead parents back or extend his own lifespan enough to meet them again when they got reincarnated, if he got more powerful. And he started doing questionable experiments, and becoming a demon.
Sasori. LOOK at this boy!◊. His parents died when he was very young and his grandmother kept lying to him that they were away on a mission. He grew so alone from the lack of love from his mother and father that—when Chiyo taught him the art of puppets—his first two creations were that of his Mom and Dad. But they could never replace the kindness, love, and affection that human parents could provide, which lead to the development of his twisted views on immortality, the sanctity of life, and how it was so fleeting and needed to be preserved... Cold, emotionless, stoic, and impatient with wasting time. Gee, thanks Sand Village!
The Kyuubi, erm, Kurama of all people...the other Bijuu too. We see them all as young beasts with the Sage of Six Paths shortly before his death, and Kurama is tearing up at the imminent loss of the man. It turns out that countless years of being treated as monsters led them to become what they are, but they've now found a human they can believe in: Naruto.
Konan. She used to be a sweet little girl, then things happened.
And as of chapter 582, if his flashback is to be believed, Kabuto.
And now it seems that the Big Bad is this trope as well. Chapter 599 is a montage of Obito Uchiha as a kid, leading up to his Heroic Sacrifice and supposed death. It then cuts to the present where Tobi's mask has shattered to reveal himself as Obito.
Even the real Madara Uchiha qualifies. We see in a flashback that, while being a bit of a loner, he was a dreamer whose sincerest desires were to protect his brother and put an end to the clan wars.
Pretty much any villain in this series whose childhood we get to see ends up being this, it's one of the themes of the series - people aren't born evil, the ninja system makes them that way.
Teru Mikami of Death Note was a really sweet kid who would always defend the weak and was a Bully Hunter. As he got older, the fact that most people seemed to tolerate bullying gave him an intense sense of Black and White Morality which was strengthened when some of his bullies ended up being killed in an accident. Once he got his hands on the titular Artifact of Doom, he ended up as the craziest person in a series filled with some fairly unstable people.
To a degree, also Misa Amane. Had her parents not been killed by a burglar when she was a little girl, she would've possibly grown into the same beautiful and popular Idol Singer she was in canon... but without the Ax CrazyYandereness.
Light Yagami. Nice, normal family, top of his class, popular with other students... and he's killed God knows how many people by the end of the second episode.
Light and Misa were distinctly never normal; they have emotional impairments of some kind. It's possible Misa developed her psychopathic tendencies somewhere between her parents' deaths and meeting Rem, given she's had actual trauma and a showbiz career to mess with her head, but Light is pretty...empty.
Or not—the fact that he dropped tennis because it was pointless and several other moments indicate that, even if he was always broken inside, it had been getting worse before he found the titular Artifact of Doom.
When Light temporarily forgets about the Death Note, he returns to being a nice, morally sound person. In other words, he was just a REALLY bad case of With Great Power Comes Great Insanity.
Simply one person: Doctor Jizabel. If you've read Godchild, especially volume 7 and more especially the Misericorde chapters you know he's a perfect example.
Lampshaded in Lucky Star, when Konata and Kagami talk about how when criminals are arrested in real life and people who knew them are interviewed, they will always say things to the effect of, "he wasn't a bad kid." Kagami then says that if Konata ever gets caught, she'll tell the media that she had "always figured she'd do something like that".
Most of the Egrigori experimental children were like this, before whatever processes they were put through were completed. The Keiths mostly became bloodthirsty psychopaths who had no problem killing anyone they had to and Alice developed a dark side to her previously sweet personality after being shot and taken over by an AI. The fact that the kids were treated terribly and given almost no positive human contact could explain a lot (Keith Green genuinely grew to love Katsumi because she was the first person who was nice to him.)
This is a prominent theme in Code Geass, while some characters like Rolo are proven to always have been killing machines since childhood (and often, like in Rolo's case, thanks to the brutal training and brainwashing coming from the Geass Cult), loads of characters started out like sweet kids even up to the Emperor but also including, C.C., Suzaku, possibly Cornelia and Clovis, Karen and Lelouch Lamperouge.
Fist of the North Star: There was a little boy who loved his adoptive father, a martial arts master. Then his father decided to take in some students and teach them his art. The boy felt left out, especially when his father refused to teach him. The boy pushed and begged until his father decided to teach him as well... and that little boy grew up to be Jagi. The story doesn't have a happy ending.
There was a tiny baby, abandoned by his parents and adopted by another martial arts master. Though trials were difficult, and the regiment strict, never once did the little boy feel that it was a burden, being driven by nothing more than his love for his adoptive father. Then came the time, when he became 15, to inherit his master's style after a final trial and become and adult. The little boy's name was Souther, and neither does this story have a happy ending. Well,almosthad.
There also was a young boy, left orphaned with his younger brother, who knew that crying will not make things better, and instead aspired to become stronger along with his brother. As the two get adopted by a martial arts master, the two finally learn to become stronger and this young child aspires to greater glory in his quest for strength. This boy grew up to be Raoh, but his story does have a happier ending.
There's also his sister, resident Yandere Belarus. We see her in her, Russia's and Ukraine's backstory as a cute little girl who clings to her brother and pouts to her older sister. And then, we see her now, as a scaryStrange Girl in a frilly Elegant Gothic Lolita dress...
America from England's perspective, who used to be a sweet boy and just adored England. Then, we all know whathappenednext.
England too, apparently. The youngest we've seen him was in the Passing Through The Year 1000 strip, when he happily agreed to let France pretend to have conquered him. From there he became a very unhappy child, then a "juvenile delinquent" who's hobby was piracy, and eventually settling into the JerkassTsundere we all know now.
The main character Jae Hyuk of the manwha Immortal Regis started out as a sweet Ordinary High School Student devoted to caring for his Ill Boy little brother. Then both of them got dragged into another dimension full of swords and sorcery and monsters. Jae Hyuk is forced into the role of The Hero while his little brother eventually becomes the host of the Big Bad. Over the course of the series Jae Hyuk becomes more and more jaded and bitter about having his old life ripped away from him (being turned into an undead abomination didn't help either). In the sequel series Cavalier of the Abyss which takes place years later, Jae Hyuk has become the new king of the other world. The first thing we see him do is ruthlessly order The Purge of a village of suspected rebels — men, women, and children alike are all put to the sword. He also chops off the arm of a soldier who questioned his orders. He's not so sweet anymore.
Adam Blade from NEEDLESS, who is, to put it mildly, a violent LoliconAnti-Hero. Even his foster-father wonders just what had gone wrong in the process. This is actually a plot point that is currently on-going in the manga.
Chouji Suitengu, back then known as Ueno Takeshi from Speed Grapher was a pretty sweet kid, who loved playing games with his little sister. Then his parents were murdered for not being able to pay their debt to the Roppongi Club. Suitengu was sold off as a child soldier, almost killed during his final mission, and then turned into a Frankenstein stitch job. Nowadays, he's a textbook example of Four Eyes, Zero Soul.
Yuno Gasai from Mirai Nikki wasn't always such a psychogirl. When Yukiteru and Minene go three years to the past it turns out that despite her abuse, she still was hopeful and idealistic.
2nd World Yuno: When did I become so twisted?
Balalaika from Black Lagoonused to be a pretty, young and seemingly friendly girl, whose dream was to do her family proud in the Olympics as a sharpshooter. That is...until the Afghanistan War.
Surprisingly, Guts from Berserk was actually a sweet and innocent kid. The downfall started when he was three and his foster mother Sys died of the plague, leaving him to be raised by his sadistic and hateful foster father Gambino. Even after being trained for war and serving on the front lines, he was still a pretty innocent kid, until the night after his first battle when Gambino sold him to another soldier for three silver coins, a soldier who liked little boys in the wrong way, resulting in a traumatic rape despite Guts' best efforts to fight the man off. The following day when Guts coldly murdered the man was the point of no return. When he was finally forced to kill Gambino in self-defense and was driven out of the camp by a mob intent on killing him, he became bitter, hateful and cut off from other people.
Actually, The Reveal with Vash is that he briefly cracked up and believed Humans Are Bastards and displayed a really creepy suicidal laugh, after the Tessla thing. He just pulled himself back together for Rem after accidentally stabbing her, and has somehow soldiered on ever since.
Manga Legato gets a dubious entry, because while there is nothing innocent about his flashback (he was a kept catamite who was discovered in a plan to kill everyone he'd ever known and was therefore being raped to death when Knives turned up and coincidentally sliced up everyone but him, because he subconsciously used his psychic powers in self-defense), it's practically impossible not to get choked up at the look on his face when he realizes Knives is going to allow him to live and follow, and he admits he has no name.
Kiriha Aonuma from Digimon Xros Wars is shown to be gentle as a child in his back story. His father critiques that Kiriha must learn to be stronger, which leads to him being a Jerk Ass. He gets better as well.
Sakurako aka Youya from Sakura Gari was a sweet and adorable child, right until she witnessed the death of her mother under some very screwed up circumstances which turn out to be even more screwed up than they appear. She then became a seriously Creepy Child, eventually revealed to be a total Yandere.
While the primary casts of Puella Magi Madoka Magica are kids by most definition, Sakura Kyouko used to be a sweet, good Japanese Christian girl who only wanted to help her father in spreading the word of his God. Unfortunately, when Minister Sakura figured out that Kyouko used magic to bring people to his church, he snapped, killed his family, and then committed suicide. Only Kyouko survived. No wonder she's such a misanthropic Jerk Ass. Also, Homura used to be Adorkable, but fighting the scheme of an Eldritch Abomination in a Groundhog Day Loop warped her into a cold, lonely Anti-Hero.
Faye Valentine of Cowboy Bebop actually used to be a very sweet idealistic child from what could be told from the lost tape found from a time capsule. It is a very sad comparison to how she is grown up.
A Pokémon manga called How I Became a Pokémon Card has a chapter on a boy. He's a sweet boy who takes in an injured Persian who ran away from a group that keeps "precious" Pokemon to stare at all day. Who's this boy? The infamous Yakuza boss, Giovanni.
Mewtwo was actually born emotionally well adjusted; looking at the Birth Of Mewtwo radio drama, it's somewhat of a surprise to see him happily smiling and having a cheerful outlook on life. Then, of course, he meets the humans who created him and finds out that they just see him as a guinea pig; the experience turning him into a vengeful Woobie, Destroyer of Worlds in Mewtwo Strikes Back. He calms down at the end of the film and by the end of Mewtwo Returns, while he's no longer the happy youngster he was, he's certainly nicer than he was during his first few months in the outside world.
Enchu of Muhyo And Roji used to be quite nice and hard-working in comparison to Muhyo, and especially cared for his sick mother, hoping to become an Executor and support her. Losing his mother to her illness and his chance at getting the position resulted in him turningevil. After realizing the truth, that Teeki killed his mother in order to take advantage of his potential he reverts to his former personality, continuing his Magical Law studies even while incarcerated.
Also applies to Genkaku of all people, when it revealed that he was once a kind forgiving buddhist priest who was constantly bullied and beaten and once fed an injured cat. It was until the earthquake that he lost his sanity.
Barnaby Brooks, Jr. from Tiger & Bunny was a joyous, trusting child constantly showered with love and care courtesy of his very affectionate parents. But they were murdered right in front of his eyes when he was just four years old, resulting in Barnaby becoming cold, cynical, consumed by the desire for revenge and reluctant to trust anyone; up to the point where he has avoided close relationships with other people for 20 years. Subverted towards endgame in that he regains traces of his original personality upon befriending the show's Idiot Hero, Kotetsu.
Gundam SEED: Compare the Athrun in the flashbacks, who believed that Earth and ZAFT would never go to war, with the bitter, cynical soldier we all know and love. Also, it seems that the entire universe is bent on turning main character Kira into one of these. He gets it back mostly.
Gundam AGE is basically about how Flit Asuno turned from an idealist Kid Hero who wants to save humanity into a genocidal crusader who wants to exterminate every man, woman, and child on the other side. There are several flashbacks to the idealistic boy he used to be. He's pulled out of it at the very end, when he's in his sixties.
Raven/Gilbert Nightray of Pandora Hearts was absolutely adorable as a child. Following his Break the Cutie event he became a brooding, serious man with major issues. He can still act sweet without meaning to sometimes, though.
Oz could count as this, too. Even when you consider the fact that he used to be this innocent little stuffed rabbit who was turned into a Chain of destruction against his will.
Nanami of Revolutionary Girl Utena. Also, in episode 9 there is a back-story to Saionji's childhood depicting the origins of Saionji's current controlling, depressed Jerkass personality. When he was younger he was more easygoing and kinder.
PsyrenBig Bad Amagi Miroku 'Grigori #06' is an extremely homicidal Magnificent Bastard who decided to Kill All Humans! except those with Psychic Powers who are willing to help with the plan, after spending most of his childhood and adolescence as a government test subject. However, several years into being a test subject, while his twin #07 was nonresponsive and #05 had already defaulted to hate, #06 was still the sweetest kid. The fact that he held onto the hope of living as a human being the longest seems to have enhanced the virulence of his hatred once he gave up.
LaxusDreyar was once a cheerful young boy who idolized Makarov, his grandfather and guildmaster of the title guild. Due to the influence of his father Ivan, and his growing resentment at his accomplishments being compared to Makarov's, he grows more and more bitter until he finally tries to take over the guild by force and remake it in his own image, the final part of his plan being to beat Makarov in combat. He never gets the chance to fight Makarov, his plan being thwarted by the guild as a whole and Natsu and Gajeel team up to defeat him. Later averted when he returns after being expelled from the guild, as a much kinder and less prideful person.
Accel World has Nomi, a.k.a Dusk Taker. In the last two episodes of the anime, it's revealed that his mentor, who introduced him to Brain Burst, used and abused him to the point of breaking. When he's defeated, and the Brain Burst (and his memories of it) erased, he reverts back to being a cute boy.
Yu-Gi-Oh!'s monsters tend to have their own stories. In the case of the charmer monsters, Eria is hinted to grow up into Gishiki Ariel. It is unknown whether or not this was of her own free will or if she was brainwashed like other Gishiki monsters. However she went from sweet little girl into ruthless warlord.
Believe it or not, Queen Nakia from Anatolia Story fits in perfectly. The homesick and scared teenage girl forced to marry an much older man as a part of an alliance in the backstory... well, she grew into a massive Manipulative Bitch and Hot Witch, who is also the Big Bad of the story.
A Cruel God Reigns: Jeremy, Jeremy, Jeremy... At the very beginning of the series, he is sweet to the point of being ridiculous, but after he is physically and sexually abused by his stepfather for 6 months, he finally snaps and kills him via Vehicular Sabotage. Actually, it is Jeremy's sweetness that causes his Trauma Conga Line in the first place; He refuses to seek help because it would mean ruining his mother's happy marriage. Although more of a Conflict Ball with Fille Fatale problems than evil, he never really snapped back. Arguably, Jeremy still is sweet kid- he just has some issues he needs to work through.
What about Batman? He's not evil, but he sure is dark.
There are a couple of Star Wars Expanded Universe webstrips set between the original and prequel trilogy. They cameo Ysanne Isard, Big Bad of the X-Wing Series, as a little girl. She idolized her father, Director of Imperial Intelligence, back then, although her innocence was in doubt even then, since he brought her to see his work, which involved hunting down and killing Jedi. As she grew up she was groomed to be a field operative, and started cultivating connections, which worried her father to the point that he sent her on a suicide mission. She survived, came back, trumped up a treason charge, executed her father, and took his place.
Herr Starr, a Big Badnote It's hard to pin down just oneBig Bad in Preacher; candidates for the title include God from Preacher was this, until a bully-induced trauma that cost him an eye, his hair, and a good chunk of his humanity, before he'd even hit puberty.
The Garth Ennis penned tale "The Tyger" shows a snippet of the childhood of Frank Castle (aka. The Punisher). He's a quiet, thoughtful young man who does well at school and is quite close to another girl his age. Events of the one-shot however show that Frank was surrounded by the refuse left by mobsters from an early age, and it left its mark.
The Incredibles: As an adolescent, Syndrome aka Buddy Pine was an adoring fan of Mr. Incredible until he was seemingly brushed away by his idol.
In Toy Story 3, Chuckles said of a young Lotso "He used to be such a good toy." That is, until their owner Daisy accidentally forgot about them while on a picnic. After a grueling journey, Lotso returns with Chuckles and Big Baby to see Daisy with another Lotso-Hugging Bear and becomes cold and bitter, convincing the others that Daisy never loved them.
Eight Crazy Nights: Davey was a teen basketball star and liked by practically everyone. Upon learning that his parents died in a car accident on the way to one of his games, he grew up to be an alcoholic jerk.
Film - Live Action
Star Wars: Anakin Skywalker. Innocent bright eyed mother's boy who builds robot buddies turned into well... Darth Vader.
Shown mostly in the novelization, but Boba Fett turned from a sweet kid who idolized his father into a bounty hunter who had to be reminded to curb his penchant for disintegrations.
Subverted, or possibly entirely inverted, with Eddie from The Rocky Horror Picture Show, if we are to believe his dodgy scientist uncle, Dr. vonScott: "From the day he vas born, he vas trouble/he vas the thorn/in his Mutter's side"
Judging from the flashback, Loki from Thor had a good relationship with Thor and his father and had a penchant for mischief before the jealousy settled in.
Loki was much the same in original Norse Mythology, sans the jealousy issues. Thor and Loki were frequently involved in mischievious pranks, and nobody thought too much of Loki's shenanigans until one of them got Baldr, the incredibly popular god of beauty, killed.
Godzilla Final Wars even originally was going to (and the final cut even heavily implies) that incarnation of Godzilla is Junior.
In Jonathan Carroll's novel Bones of the Moon, the main character's husband comments that the boy who lived in the apartment above them, dubbed "The Axe Boy", was always such a good boy before he was arrested for chopping up his mother and sister.
"He seemed like a good kid, didn't he Cullen? 'Axe Boy'? Jesus, what a thing to call someone!"
"Danny, our young friend 'Axe Boy' Alvin Willians chopped his mother and sister into pieces exactly one floor above our apartment. A good boy he is not."
Subverted in two ways with Sirius Black: he's been a pretty nice guy all along and he was also an immature, arrogant, bullying rebel as a teenager, averting the cliche that all good people are flawless angels as children.
Snape seemed to have been about as good as a kid raised in an abusive and poor household could be expected. And while he eventually did a Heel Face Turn to Dumbledore in his twenties, there was a period of time when he served Voldemort and remained bitter enough that many people believed he still was evil into his thirties.
And Peter Pettigrew must have been at least a good and moral person, for the Marauders to both be friends with him and to trust him with their lives as adults.
Will Graham: As a child, my heart bleeds for him. Someone took a little boy and turned him into a monster. But as an adult... as an adult, he's irredeemable. He butchers whole families to fulfill some sick fantasy. As an adult, I think someone should blow the sick fuck out of his socks.
Warrior Cats: Tigerstar, big bad of the entire series, is seen in the prequel Bluestar's Prophecy as a sweet, innocent, adorable kit who loves his mother and grows into a well-meaning but aggressive apprentice thanks to his Axe Crazy mentor. Same with Scourge, who in his backstory was an inquisitive, cute little kitten who had bad things happen to him, turning him into a crazy killer.
From Aunt Dimity Down Under: The expat Americans Angelo and Renee Velesuonno knew Broken Bird Bree Pym since she was ten years old, and Angelo hangs this lampshade when they meet Lori and Cameron for dinner in Ohakune, New Zealand. His description of Bree: "Nice kid—good manners and sharp as a tack." The Velesuonnos go on to say they were shocked at the recent changes in her appearance, particularly the choppy haircut.
The Mord-Sith from The Sword Of Truth series. Every year, six of the kindest, sweetest girls in the entire country are chosen to be trained as Torture Technicians. The kinder they are, the better.
In Torn by Cat Clarke, it's revealed that Tara used to be an extremely sweet, kind young girl until when they were younger, Alice, then Tara's best friend, grew irritated with how she and Tara were the least popular girls in school and purposefully started to avoid her in an attempt to join in more with other kids. Of course, it backfired horribly.
Harry Enfield's Kevin was a well-behaved, if hyperactive, boy until he turned thirteen, at which point his hormones kicked in and turned him into the teenage horror we all know and love.
Beverly Hills 90210 character Valerie Malone, revealed during the civil rape suit she files against Noah by Steve.
Law & Order: SVU episode "Swing", Elliot Stabler's interaction with his mother highly suggests that he was a sweet kid with a dream to become an architect. His mother even suggested that he's turned out like his father.
In Kamen Rider Double's movie, Big Bad Katsumi Daido/Kamen Rider Eternal is described this way by his mother. He's an unusual case since he died and was brought back to life by Mom's research, which may have contributed to his turn.
Doctor Who: The Master was apparently a sweet kid until he looked into the Vortex at age 8, and heard the drums. Then he slowly went batshit insane.
Invoked by name in "The Rebel Flesh"/"The Almost People", except with reference to the sweet kid's artificial duplicate.
Debbie Pelt from True Blood. It's almost painful to watch a flashback that includes the sweet, well-intentioned girl you know will become a V-addicted nutcase.
Once Upon a Time Both Rumplestiltskin and Regina. Rumplestiltskin was a coward, but loved his son deeply. His efforts to protect his son were what turned him into a monster.
It's taken even further with Regina. She was genuinely good person who loved a stable boy. Her first interaction with Snow White is actually saving her, and she is later the one to tell her that "true love is the most powerful magic of all''. Then her mother murders that man she loves, an attempt to bring him back fails ( other characters deliberately sabotaged it to drive her over the edge), and decides revenge is all she has left.
Inverted with Queen Eva, Snow's mother. She used to be a spoiled brat who spitefully tripped Cora. But by the time she married and had Snow, she became a much kinder person and quick to lecture Snow's own bratty behaviour.
Also surprisingly subverted with Cora. She was already proud, bitter, vengeful and calculating prior to Rumplestiltskin coming into her life, although she wasn't evil by any stretch. The way she used her wits to her advantage and didn't hesitate to talk back to arrogant royalty is rather admirable.
Played for Laughs in Joe's Garage by Frank Zappa. Joe's neighbors viewed him as this ("He used to cut my grass, he was a very nice boy."), until he discovered rock 'n roll.
Used in Lordi's "The Kids Who Want To Play With The Dead", which is a blasting of the the old media's hatred of the new media and love of blaming them ("The must have been some dangerous toys, and the music sings of murderous ploys...")
The sweetest Girl by Nicki Minaj. "She used to be...(Yeah, she used to be the sweetest girl!) When a good girl gone she gone forever."
Kingdom Hearts Birth By Sleep includes glimpses into the lives of several Organization members before they became Nobodies Ienzo was a cute Child Prodigy who follows his elders around in an oversized labcoat, and Lea and Isa were best friends with them having the dynamic of a Keet and Kuudere flavored Red Oni, Blue Oni. Isa is probably the worst, given how Saix is one of the most cruel Organization members.
Kingdom Hearts 3D [Dream Drop Distance] has Lea and Ienzo regain a little of their childhood sweetness upon the return of their hearts... Isa however? The game reveals that Isa / Saix has been slowly turning into another Xehanort vessel, which makes it unclear how much control he's had of his own actions.
Porky Minch of EarthBound may not have ever been a "sweet kid" by any stretch of the imagination, but he was certainly a normal one. Throughout the course of the story, events slowly dragged him in and corrutpted him, to the point where he became The Dragon to Giygas, and may actually have been pulling Giygas's strings by that point. When that fell through, he disappeared into time, later resurfacing to invade and corrupt the Nowhere Islands, dismember and reassemble its wildlife, and slowly suck the soul out of Tazmily Village until everyone left for his city. All of this was done out of sheer boredom and shows absolutely no remorse for anything, believing himself to be victimised and in the right.
In the end, it's shown that Giygas and/or the Mani Mani Statue were playing on his desire to be liked, when he states one of his main motivations for destroying the world to be so that "...everyone who won't like me is gone."
Darion Mograine of World of Warcraft, as a child, loved his father and hoped to become a paladin. After sacrificing himself to save his father's soul, he became a Death Knight agent of the Lich King's will, and later, a ruthless Anti-Hero willing to do whatever is necessary to defeat the Lich King.
For that matter, the Lich King himself. The child we see early in his novel is something of an idealist, loves animals, and is kind even to commoners to a point he's said to spend more time with them than is appropriate for royalty, including being best friends with one. One dead horse, zombie plague, and a few questionablewar decisions later, and Arthas has become first the champion and then ruler of death itself. Special attention is even brought to this with the five-year perpetual dream he had during his long sleep after taking up the crown, in which he finds two figures battling to influence him: Ner'zhul (the previous Lich King) and his own innocent child-self. Eventually, he kills them both.
Ayane of Dead or Alive, according to Dimensions, used to play with her sister Kasumi all the time, before circumstances related to Kasumi being favored, then leaving the village to get revenge for her brother caused her to become embittered and hostile toward her.
Inverted in Dragon Quest V; Harry is a vile, abusive spoiled brat as a boy, but years of hardship and suffering turn him into a kind, honorable and selfless young man.
Played straight with Terry in Dragon Quest Monsters and Dragon Quest VI. Life between the two was rather harsh between the two games (highlights include an evil king forcing his beloved sister into prostitution), turning him from an adorable heroic figure into a darker antihero.
Here's a fun one for you. In Kingdom of Loathing, once you Ascend, you can gain access to a Temporal Rift and visit the past version of several Nearby Plains areas. One of them is Fernsworthy's Hut, the past version of Fernsworthy's Tower. Fernsworthy himself is still alive at this point, and is teaching an Ethics of Magic class. You know that cute schoolgirl who winks at you as you're leaving that scene? If only the Naughty Sorceress had paid attention in that class...
Archer from Fate/stay night, though in a subversion he's just as broken inside now as he was when he was younger — he's just grown more outwardly cynical and self-aware towards his inner failings. Fate/hollow ataraxia shows this to be the case with Gilgamesh, and Fate Zero with Kiritsugu as well.
In Ryukishi07's other famous series, Umineko No Naku Koro Ni, the main antagonist, the seeminglyAxe Crazy sadistic Beatrice was once a timid young servant named Yasu who wanted to learn magic to spice up his/her not really nice life. While Beatrice isn't actually as sadistic as she seems, she still has a lot of issues.
Akane Kurashiki, aka June in Nine Hours Nine Persons Nine Doors. She used to be a sweet girl, but the trauma of the first Nonary Game turned her into a person willing to do anything for the sake of the cause - whether that means sacrificing her life or someone else's. Junpei remarks at the end of Virtues Last Reward that the Akane he knew is gone.
Averted in the case of Xykon - the whole point of the Start of Darkness prequel book was to show that he had no redeeming features whatsoever, and never did- in the creator's words, not only is he thoroughly evil, he's also sort of a dick. While he may not have been evil when he first discovered his gift for Necromancy at the age of 4, by the age of 12 he's killing a Captain Ersatz of Professor Xavier (granted, one who was being an incredible dick about how wizards are inherently better than sorcerers), zombifying him and siccing him and his zombified grandma on his parents.
Played straight with Redcloak and his close-knit family, though.
Averted with Lord English. His younger self Caliborn is almost as bad as he is now, just smaller and constrained.
Kurloz Makara seems to have been a pretty nice normal teenager until he had some kind of Eldritch Abomination nightmare, mutilated his own mouth, and started working for the abovementioned Big Bad. His dancestor Gamzee similarly was fine until he ran out of sopor slime; it's likely he would still have turned to working for Lord English, but would have done so more subtly and without murdering Equius and Nepeta, had he never been on the drugs.
Sometimes used in Chick Tracts. Examples include Bruce in "Fallen" and Harry in "Fairy Tales".
Done tearjerkingly in The Nostalgia Critic's "Commercials Special". When he's on the edge of a Despair Event Horizon, he picks up a photo of himself as a kid and angsts about how he had so much promise and dreams back then. So that sweet, "perfect" child went through a Break the Cutie (parental abuse, date rape, bullying and stalkers) and became a cynical, depressed man who has nothing left in his life but reviewing. He gets a Crowning Moment of Awesome right after, but still... ouch.
The Nostalgia Chick used to be an adorable girl who wanted a fairytale wedding when she was six, had quite a few friends and fell in love with a dragon because he was honorable. Then bullying, abuse, becoming a Bratty Teenage Daughter and turning to alcoholism happened, so now she's a lonely, psychopathic Broken Bird. Again, ouch.
Happens frequently in Survival of the Fittest if you read pre-game. Many characters come off as likable, friendly people, only to be broken once the game starts, in a good portion of cases turning Ax Crazy. As a result, you will be seeing this trope a lot, especially if the character showed up in pre-game frequently or there are any flashbacks involving the character.
The Doomy Adventures Of Irken Doominess: Melissa as a smeet was a very fraile and kind irken, until she was abused by her classmates, lost her sanity, began killing lots of people, got a taste for human blood and over all became quite messed up to some degree.
Gar who was normal as a smeet, got ridiculed by everyone, was singled out to be the tallests personal Chew Toy, started Hearing Voices, got a major Sanity Slippage which resulted in her trying to kill the tallest.
Deef might qualify since Word Of God states his mother cared about him up until the age of six when she decided he was able to take care of himself, he still lives with her but seems neglected by her, although Deef isn't insane or there isn't anything seemingly wrong with him, he's strange and has No Social Skills.
In Thor Tales Of Asgard it was revealed that Loki was like this as a teenager. Its not until the betrayal of a family friend and the use of Sword of Surtur that his former outlook starts souring and he begins to think different.
Monty Burns on The Simpsons was depicted as one in "Rosebud." A flashback showed him happily playing with his teddy bear Bobo, and being referred to by his parents as "Happy." He immediately abandons them, however, to live with a "twisted, loveless billionaire."
Similarly, his younger brother, Tarrlok, was a very sweet and timid boy. The adult version...NotSo Much.
In the Teenage Mutant Ninja Turtles 2003 episode "Insane in the Membrane", it appears like this trope will be subverted when Baxter flashes back to his childhood and is seen about to pour some sort of chemicals on a bug in a jar. He changes his mind and lets the bug go. Through more flashbacks, it was clear that he was a good kid who was devastated by his mother's death, and the memories of her were enough for him to let the Turtles and April go.
Zig-zag: In the 1993 cartoon Itsy Bitsy Spider (based on the animated short subject that played with Bebe's Kids in theaters), Leslie—the little girl who befriends Itsy—was a sweet, introverted kid in the short and first season. Many episodes in season two have her a head-slapping bitch.
Star Wars: The Clone Wars: Asajj Ventress, through a series of flashbacks, is shown to have first been a cute little girl who later turned to the dark side after her Jedi Master, Ky Narec, was killed.
Archer: The title character was a fairly normal kid, but his mother wasn't around at all until he was 5, and messed him up even worse after she returned.
Ricky Owens from Scooby Doo Mystery Incorporated went from the Shaggy of the original Mystery Inc. to the devious bastard he's known as in the present. Mr. E.