In YuYu Hakusho, the Big Bad antagonists in seasons one and two are shown to have become villainous due to major psychological trauma leading to crises of faith in their formerly-held ideals (the massacre of Toguro's students by a youkai and Sensui's exposure to the depravity of humanity and then the Black Chapter tape). And in both cases enough self-hatred to stage really stupid, selfish Thanatos Gambits.
Mazinger Z: Hell's mind was already unstable and unsound cause his upbringing (he was abused by everybody when he was young, including his parents. Particularly her mother often spouted she never wanted having children and she would be better off without him; and she was not above of hiting him for no reason). However, when he found out the woman he loved was in love with another man Juzo Kabuto, future builder of Mazinger-Z and Kouji's grandfather, he... flipped out and attempted to murder his perceived rival. Shortly after he tried helping someone... and he got the crap beaten out of him for it. Later he was crawling back towards his home, bruised and blood-stained, muttering "Mediocre imbeciles! You don't deserve being alive! One day I'll purge the world off all of you! And then everybody will have kneel before me". When you heard his words and saw his utterly mad stare you realized he had snapped out completely and Dr. Hell had been born.
Let's tell people had shunned him out of scorn before, but after that point they did it out of FEAR.
The Rozen Maiden Ouvertüre OVA explores the start of Suigintou's fierce rivalry with Shinku and the source of her massive inferiority complex that motivated her to become the first season's Big Bad.
"Two Father's Little Soldier Girls", the 21st episode of Black Lagoon contains a number of flashbacks to Balalaika's past detailing her childhood, service in Afghanistan, disillusionment with society and rise to power in Hotel Moscow.
Almost every Rave Master villain gets at least a chapter for this. Some are a little sad, some will leave you temporarily cheering for the villain (until you remember that every single one aims to wipe out all life as we know it).
One episode of RahXephon presents the back-story of Makoto Isshiki, an Evil Albino who is a cold-hearted seducer and major jerk to everyone else. It shows him as a cute and kind boy who just wanted to find his parents. Then, one day he admitted to himself what he really was, and his flashback ends with him getting an Important Haircut and taking on his nasty personality.
One Piece has Chapter 0, a special tie-in to the tenth movie. In it, we see Shiki and Gold Roger's rivalry, as well as the Flying Pirate's escape from Impel Down and his preparing for the movie's plot. Interspersed are scenes of people throughout the world, reacting to both Roger's death and Shiki's escape.
To a lesser extent, the Fishman Island arc explains part of what made Arlong who he is.
Naruto has done this with many antagonists so far, and attempts to justify the Uchiha examples, by claiming that their powers take hold of them because they love too much:
Deuteragonist Sasuke Uchiha witnessed his brother Itachi murder their parents, after murdering their entire clan. He then was subject to days of Mind Rape compressed into seconds, and a challenge to kill his brother when he's ready. None of this makes him "evil", so much as focused on killing his brother, a world-class criminal, on his own. After months of being shown up by his rival, Sasuke gets reminded by Itachi again that he's insignificant. He then follows Orochimaru, the series' first Big Bad, to gain the power to beat Itachi. After he finds out the truth about Itachi's backstory, that it was the clan itself that was evil and Itachi was just following orders, he begins to show signs of mental imbalance, playing into the hands of the new Big Bad manipulating him, Tobi, and he decides his best option is revenge on their entire home town.
Gaara grew up believing his mother named him as a curse against everyone else, for her having to die in childbirth. His father had warriors and himself to keep Gaara in line, because of the demon sealed within him, which would be unleashed if he lost control of his emotions or even slept. As a small child, he believed that there was only one person who loved him, his maternal uncle Yashamaru. The betrayal of Yashamaru transformed him from an attention-starved waif to serial killer. He came around to the side of good once he realized what friendship was, long before any of the above was actually fixed, and years before it was all retconned to be a big misunderstanding, and his parents always loved him.
When Sasuke "defeated" Orochimaru the manga briefly touched on Orochimaru's past, while the anime expounded on it. His deepest dream was to live long enough to meet his parents when they were reincarnated. Years of war and loss turned him cold and bitter until only the dream of living forever remained.
Nagato lived in a state of constant war, as his homeland was the battleground for all the bigger countries' wars. For years, he was subtly manipulated and watched by Obito as Tobi, and Madara as Black Zetsu. Then, his parents were killed in front of him. Eventually, the death of his dear friend Yahiko and his subsequent crippling drove him to share his Pain with the rest of the world.
Kakuzu was imprisioned and punished for failing to kill the First Hokage. As a result he steals the elders hearts and runs away.
Kabuto grew up an orphan, but that didn't let him get down. However, he never really knew who he was meant to be. He was inspired to be a ninja by the first people to challenge him to grow in his natural talents, Orochimaru and Danzo. While on a mission for them, he was tricked by them into accidentally killing his foster mother, who in turn had failed to recognize him due to their brainwashing of her. After this happens he hits the Despair Event Horizon and Orochimaru was the only person around that was there for him at that point. He commits some good, and a lot of evil, actions for the rest of the series, but only comes into his own once Orochimaru dies and he no longer has any guidance or kindred spirit.
Obito Uchiha, after some subtle mental influencing by Madara, witnessed first-hand the death of his childhood crush at the hands of the friend who vowed to protect her and became convinced the world he lived in was hell. He then decided to accept Madara's plan to create a better world by killing or brainwashing everyone in the world.
Madara Uchiha lived through a time of warfare, like everyone else in his generation. Then, thanks to his best friend's work, which he attempted to co-opt and sabotage, things started getting better and he mostly got over the wars, like everyone else in his generation. Then this one time, he didn't get to be the top boss of the country, after he eavesdropped on a private conversation between his best friend and his best friend's brother, where he found out that not everyone liked him, because he was such a bully to other people, his own family, and the people they were supposed to be courting as allies. So instead of being happy for his friend who did get the job, he used an ancient stone detailing the world's constantly warring history to assume said friend's system would fail and justify his becoming the Big Bad and trying to brainwash or kill everyone in the world, despite that in no way saying he in particular should take any action. Ladies and gentlemen, your final villain.
His mangareason made Vash try to kill himself twice and laugh hysterically when he thought he'd accidentally killed Rem! And manga Knives Used to Be a Sweet Kid—more than Vash, even. Some of adult Vash's ideals are stuff Knives said as a kid.
Legato also gets one of these, in the manga. Unusually, it has a very upbeat ending — because psychotically serving an Omnicidal Maniac who doesn't really give a damn about you is so much better than being raped to death by the guy who owns you. Knives didn't just spare him, he even asked his name! Weirdest Crowning Moment of Heartwarming ever. And nameless-boy-who-would-become-Legato goes stumbling after him naked and weeping for joy.
Livio has one, too, although his is complicated by the fact that first he got a psychotic alternate personality, and then later he was acquired by the Eye of Michael and turned into a killing machine in his own right.
Anime Wolfwood also gets one. Manga Wolfwood just gets back story snippets. But because the anime is Lighter and Fluffier, Wolfwood's philosophical position has a lot less pull there, so he needs a more clear-cut 'reason' to think the way he does.
Romeo X Juliet practically drops it line for line with the episode "Darkness: The Origin", which gives Prince Montague a somewhat run-of-the-mill Freudian Excuse.
Subverted in episode 10 of Puella Magi Madoka Magica. It seems to be one of these for Homura, but instead, it ends up showing that she's far less villainous than she seems (all of her Anti-Hero antics are an attempt to prevent Madoka from dying and/or suffering a Fate Worse than Death). Played Straight for Kyoko in episode 7.
Shu: "I was wrong. Kindness is pointless. We have to separate the good from the trash. I'm going to become king." Heil Mein Shurer
Death Note After Light realizes what he posses, he decides that he enjoys having the power to kill those he views as evil. He even jumps the gun and starts his God Complex after he finally meets Ryuk and tells him about his intentions.
Yet another Start of Darkness for the Master in the Big Finish Doctor Who audio drama Master. Apparently, it all goes back to when he and the Doctor were at the Academy, and he killed an older boy who was tormenting them. This isn't what happened. The Doctor killed the boy and then made a deal with Death for the guilt to be transfered to his friend.
The audio drama series I, Davros shows the early life of everyone's favourite Dalek creatingMad Scientist. Interesting in that he isn't given any Freudian Excuse, and you don't gain any sympathy for him, just understanding.
The issue Mad Love of The Batman Adventures is also partially a Start of Darkness story, this time for Harley Quinn. It explains who she was and how she ended up with the Joker.
A large portion of Gold Digger's 75th issue was dedicated to Gina and company discovering the origins of both Alfred Peachbody (who was less noble in his beginnings) and Dreadwing, who fits this trope to an absolute T. Though Dreadwing's origins are detailed as mentioned above, it's further fleshed out in the Dreadwing's Myomior special, which is essentially Dreadwing's past narrated by himself. Arguably effective despite the repetition, as the unbiased depictions of events from earlier serve to highlight the twisted perspective Dreadwing views his past through. It manages to make him sympathetic and tragic, while simultaneously keeping him a Magnificent Bastard in the present.
The Saint of Killers miniseries, which explained how the titular killer became the Implacable Man he is in the series proper. When he first appears, we know he was already a killer in the Civil War, but not how he got that way. Part of the miniseries shows his softening and becoming a family man. However, after a delay due to ruffians led to his family dying of fever, he returned to his killing.
Herr Starr, the Big Bad, has his own issue of this, as well, showing how he rose to the position that readers see him in. He gets a Pet the Dog moment and a legitimate claim to having been a good guy at one point — and quickly shows the predilections that make him such an outstanding villain for the rest of the series.
While the Knights of the Old Republic comic series is mostly Zayne Carrick's story, in its background it deals with the Jedi duo of "the Revanchist" and Alek, showing their gradual transformation into the Revan and Malak we see in the game. In fact, this trope is so pervasive that Zayne himself and his Master Lucien were speculated to be a past version of every Sith Lord from the two games at some point.
The Judge Dredd supervillain Judge Death has his origin given in "Young Death — Boyhood of a Superfiend". This shows (with some incredibly black humour) how a nasty and psycopathic child develops into a monster that wipes out his whole world. (Although, to be fair, the reoffending rate is to all intents and purposes negligible.) Darkness hardly begins to describe it....
Transformers: More than Meets the Eye: Has the Shadowplay arc, which is being told to a comatose Rung to help Jog his memory. It's about Orion Pax (Optimus Prime) fighting down the corrupt Pre-war senate who seek to kill a bunch of bots, with the help of a non-corrupt senator and ragtag police crew. At the very end of the arc it's revealed that it's the Senator's start of Darkness, taking the fall so Optimus and his partner get out, and having his emotions destroyed and appearance altered forever, and this was the story of how Shockwave came to be who he is now.
The gradually unfolding back story of Winnowill and Two-Edge rather took over the second major arc of ElfQuest. We got to see exactly how and why both ended up such raging twisties.
X-Men villain Magneto has a truly long and harrowing SOD that was revealed in snippets throughout the years following his debut and would be truly over-the-top if everything about it wasn't mostly grounded in reality. It's far too long to fully recount here, but let's just say that it isn't too surprising that someone who witnessed first hand the evils of Nazi Germany, the Holocaust, the Soviet Union, and Red Scare stricken America while losing his whole family, a girlfriend, a wife, and a daughter along the way as well as finding out he's a mutant in the Marvel Universe would end up a Well-Intentioned Extremist.
"Tales of the (X) Corps" strips in Green Lantern and Green Lantern Corps show the Starts of Darkness of various Sinestro Corpsmen and Red Lanterns, as well as Agent Orange. Some of them have tragic pasts, others (including Larfleeze) just aren't very nice people.
All Red Lanterns have tragic backstories—at the very least they were betrayed by their fellow gangmembers—because the red power comes from rage, hate, and resentment.
Captain Marvel: Black Adam's turn to darkness was explored in a trip to the past. He used to be a champion to his people called Mighty Adam and was every bit the hero. Then a supervillain killed his family. He hasn't been the same since. History repeated itself in 52.
The Pony POV Series has the Origins arc, which shows the backstory of Celestia, Luna, and Discord, and therefore serves as the latter's Start Of Darkness. Discord was never quite right to begin with, as it's shown that the draconnequi were like the Shiva of The Multiverse, performing Cosmic Retcons in order to prevent worlds from ending, with Discord enjoying it far more than his siblings, who were only doing their jobs. That being said, in the grand scheme of things he was just a bully and a brat — until an act of defiance on Celestia's part triggered a series of events that not only led to Discord embracing his sadism, but began a war between the Alicorns and Draconnequi, during which he was further mentored in evil by the Fallen AlicornMorning Star. When the war ended, Discord was reborn in Equestria as a mortal (along with Celestia and Luna) with no memory of his past existence, leading to him becoming an odd, but ultimately kind-hearted being... until his true personality resurfaced and consumed his good one. From that point on, he was the Big Bad from canon and the rest of the series. Oh, and bonus points, it's shown at the end of the arc that when the Princesses defeated and sealed him away, his last act was to plant the seeds of doubt that led to Luna becoming Nightmare Moon. So he's responsible for her Start Of Darkness too!
We get to see Diamond Tiara's Start of Darkness occur as the story progresses, with the primary Break the Cutie process happening during the Mind Games arc. By the time it's done, Discord's succeeded in breaking her so fully that she goes Nightmare and becomes his Dragon.
"Fading Futures" was a fan-made ending to the (at the time) unresolved Bad Future, that involved Twilight Sparkle becoming the vengeance-crazedNightmare Purgatory and then having a Heel Realization. However, when the Bad Future was officially revisited in the Dark World Series, it was eventually revealed that the Bigger Bad is Purgatory's evolved form Paradox, at which point Alexwarlorn (the primary author) released an altered version of "Fading Futures" called "A Fading Future", edited so that there was no Heel Realization and showed Purgatory/Paradox starting her revenge plan, making it a proper SOD.
Chrysalis' start of darkness is covered as an addendum during the short arc that followed the end of Dark World: back at the Dawn of Time, a younger and more Hot-Blooded Cadence became convinced that magic was inherently evil and tried to kill Amicitia (the Magician Alicorn) in order to prevent its existence. After Amicitia nearly killed Cadence in self defense, Cadence's Light of Existence went to live a mortal life to teach her how wrong she was, leaving behind her Shadow of Existence... which, against every rule of the POV universe, somehow clung to sentience and a sense of self through sheer force of will. Pandora was impressed enough by this that she helped the Shadow gain its own Light, turning it into Chrysalis, who was sent into the mortal world.
And then there's her Origins Episode, which shows her childhood after being reborn and how she eventually became Queen of the Changelings by killing their previous Queen, Cocoon.
In the Puella Magi Madoka Magica doujin, "I'm working at a mahou shoujo recruitment company, but I think I may be at my limit" by Momiji Mao (NSFW warning for ads, since the doujin in question is on Danbooru), Kyubey is a worker at a magical girl recruitment company and down on his luck, such that his boss is threatening to terminate him if he keeps dragging down the company. The only people who believe in him are his wife Kyuko, who is three months along with Kyubey's baby, and his work buddy Kyuzo, the only one in the workplace who knows he is trying his best. Bolstered by this, Kyubey works extra hard, even pulling all-nighters, and manages to get ten new magical girls into the fold. But then it's revealed that not only did Kyuzo screw Kyubey over by taking all the credit for his accomplishments, but when Kyubey calls Kyuzo up and demands an explanation for this shit, that's when he (and we) learn that this isn't even the half of how Kyuzo's betrayed him — Kyuzo is in bed with Kyuko when he gets the call, and it turns out that he's been banging her on the side for at least three months, because that kid that's three months along? It's not Kyubey's. With everything good in his life taken away from him in one fell swoop, Kyubey snaps and goes postal on everyone before burning his workplace down. He makes a resolution that the next time he meets someone, he should give them the same cruel lesson that he believes Kyuzo and everyone else were trying to teach him.
We also see this trope in another Madoka fanfic, Stars Above a crossover with Lucky Star. Homura first explains the Start Of Darkness of her timeline's Kagami in Chapter 3, but we see in happen via a Whole Episode Flashback in Chapter 9.
In Through A Diamond Sky, Clu is a Jerkass, but not the dog-kicker of the film. At the end of the story, he's turned the torture devices from the Resource Hog base into the "repurposing" racks and uses the captured Hogs as the first test subjects.
It's also the fact that he was jealous of his creator when it came to Jordan, saw that the Isos could reproduce, and saw Kevin and Jordan being in awe of the new Iso child. Having Tron blow off his fears and scold him didn't help.
There's a Tumblr roleplay that crosses over X-Men: First Class with the alluded-to-in-canon 1960s era of Watchmen, which means that Magneto and Mystique's fall to villainy is played out right alongside, and in some cases interactively with, Ozymandias and Rorschach's own slides into Anti-Villain status.
Inner Demons is one for Twilight Sparkle, but the Queen of Darkness' legacy is revealed to go back further than that. The original Queen of Darkness was an Alicorn named Midnight who was spawned in Tartarus, but upon seeing the light and beauty of Equestria, fell in love with it and eventually became its benevolent queen. Then the denizens of Tartarus tracked her down and demanded that she become their ruler and destroy Equestria. This caused Midnight's subjects to betray her, which in turn caused her to become the Queen of Darkness and try to destroy her former kingdom, until the pony who would become known a the Master of Harmony rose up against her and destroyed her with the Elements of Harmony. With her last breath, Midnight swore to return and have her revenge, and eventually she was reincarnated as Twilight Sparkle, who in turn was therefore destined from birth to become the next Queen of Darkness.
"And so little by little the monster inside his soul would wake up. Little by little he would change his outer appearance to match what his soul had gotten dark with: rage, hate, and fear..."
"That's the Story of a man... who wasn't."
Shadows Awakening: Chapter 11 details The Phantom's origins — he was once a samurai named Kyosuke, who was the protege of the Emperor's High Shogun, but who grew unhappy with the fact that the Emperor is only ruler because of the supposed divine right of his ancestors, rather than any real power. Because of this, when he was sent to infiltrate the Brotherhood of Shadows (the dark wizards who created the Shadowkhan), they were able to convince him to join them, becoming their leader, the Dark Champion. He then led them on a conquest of Japan, until he was abandoned by Tarakudo, and struck down by his former master. With his dying breaths, his rage at Tarakudo's betrayal transformed his spirit into the Phantom.
According to his backstory, the evil Toad from Flushed Away was once the pet toad of the Prince of England, but one day the Prince received a pet rat for his birthday and flushed the Toad down the toilet. As a result Toad actually developed a hatred against all rats and wanted to have them all drown when the sewers get flooded during a soccer game due to the audience all using the toilets all at once. Ironically, some of his henchmen are actually rats.
In Kung Fu Panda, we get brief glimpses of Tai Lung's when Oogway refuses to give him the Dragon Scroll. Master Shifu was partly to blame for this as well, since he never properly disciplined Tai Lung, and instead continuously praised and encouraged him, which helped lead to his Face-Heel Turn.
Shown in a prologue in The Incredibles on how Mr. Incredible giving Buddy, his #1 fan, the cold shoulder eventually turned him into Syndrome.
A tie-in storybook based on Disney's The Lion King was actually about Scar's backstory which explains not only how and why he became the series' Big Bad, but also how and why he got his scar in the first place, as well as his real name.
Megamind narrates his own start of darkness being the villain to Metro Man back when they were school kids. Metro Man was adored by their classmates, but no matter what he tried, no one liked Megamind. So, he decided, since being bad seemed to be the ONE thing he was good at, he would BE as bad as he could be!
Toy Story 3 gives us a flashback of Lotso-Huggin' Bear being accidentally abandoned in a field on a picnic, then replaced with a new model. The flashback's narrator tell us "something snapped that day."
In Wreck-It Ralph, there was once a character named Turbo, protagonist of the racing game TurboTime, who loved the attention he got from gamers when they played his game. When a newer racing game, Roadblasters, came to the arcade and took the gamers' attention away from him, he didn't take it well. He took it so poorly, in fact, that he left his own game and entered Roadblasters in an attempt to sabotage it. He succeeded, at a price: both TurboTime and Roadblasters were deemed permanently out-of-order, unplugged and removed from the arcade. Turbo's reckless actions were so shocking that a phrase was created to describe them: "going Turbo". It's actually through an explanation of the meaning of the phrase that the aforementioned events are shown in a flashback. It turns out that Turbo actually survived and went on to invade an even newer racing game by the name of Sugar Rush, forcing himself into the game with a disguise and a new name: "King Candy". Unfortunately, the game already had a playable royal by the name of Vanellope von Schweetz, but he soon fixed that little issue. ...So, yeah, he's definitely the Big Bad.
Randall in Monsters University starts to despise Sulley after Sulley accidentally humiliates Randll in the last round of the Scare Games.
Films — Live-Action
Mike Ehrmantraut broke bad when dealing with a domestic dispute case with a routine wife beater as a beat cop. He eventually became fed up, took the husband out into the desert, roughed him up, and threaten to kill him if he hurt his wife again. Weeks later, the man ended up killing his wife, and Mike is implied to have made good on his promise. Shortly afterwards, he was let go from the force, and his life as a career criminal began.
The trilogy of Star Wars prequels, showing how the Empire was formed and how Anakin Skywalker became Darth Vader.
The flashbacks in The Godfather Part II show how Vito Corleone got to where we saw him in the first movie. When he's a little boy in Sicily, his family is killed by the mafia, and he has to be smuggled to America to avoid the same fate. He grows up to get married and work in a grocery store, apparently not planning on a criminal career, until he loses his job to the nephew of the local mafia boss and, around the same time, is lured into his first robbery by a friend. Later, when the aforementioned boss demands a cut of the proceeds from the friends' now brisk trade in stolen goods, Vito decides to kill him instead and effectively replaces him in the neighborhood. His evolution is complete when we see him travel to Sicily to avenge his family's murder. All of this is a parallel to his son's Protagonist Journey To Villain in the present-day parts of the film.
Iron Man 3 shows Killian's start of darkness after Tony doesn't meet him on the roof to talk about A.I.M. He looks over the edge and considers suicide, until he realises that no one even knows he's there.
When I was a boy, I was brought up in a circus. My only real friend was a huge, magnificent, African bull elephant. One day, his handler mistreated him and he went berserk. Bleeding, dying, he came and found me, stood on one leg, his best trick, picked me up and put me on his back. The drunken handler came along and emptied his gun into his eye. I emptied my stage pistol into his! You see, Mr. Bond, I always thought I loved animals. Then I discovered that I enjoyed killing people even more.
Janus (aka Alec Trevelyan) from Goldeneye tells his to Bond in the statue park scene.
We're both orphans, James. But where your parents had the luxury of dying in a climbing accident, mine survived the British betrayal and Stalin's execution squads... but my father couldn't let himself or my mother live with the shame of it. MI6 figured I was too young to remember... and in one of life's little ironies, the son went to work for the government whose betrayal caused the father to kill himself and his wife.
Hannibal Rising is a poorly-executed Start of darkness for Hannibal Lecter, giving him a Freudian Excuse for many of the things he's famous for, even though he explicitly stated in the first movie that there wasn't any past trauma behind his deviant behavior — making him yet another intellectual in blatant denial.
A minor example in the Star Trek: The Lost Era novel The Art of the Impossible. Corbin Entek, a Cardassian Obsidian Order villain from a highly popular episode of Star Trek: Deep Space Nine, is a lowly junior probationist in this book, albeit a promising one. The novel features a sub-plot in which he settles into the Order and earns the admiration of Enabran Tain.
Outbound Flight serves as a Start of Darkness of sorts for Grand Admiral Thrawn. Although he isn't exactly evil, it does explain why he took Palpatine's side. Eventually. Well, it introduces him to Darth Sidious and shows how perilously close he is to being exiled for his tactics. We know from the short story "Mist Encounter" that after he was exiled some Imperials found him and brought him back.
The novel Dark Rendezvous has several flashback scenes that explore Count Dooku's past and gives him a very convincing SOD backstory.
The Han Solo Trilogy by A.C. Crispin features a character who appeared first in Dark Empire, the comic book series set years after the novels but released years earlier. In Dark Empire, readers learned that he was an old friend of Han's, and also that he was willing to throw away that friendship by leading Han into a trap just for the reward. Crispin shows us in her prequels what a good and heroic guy he used to be, and eventually what happened to change him: he was captured, tortured, and crippled for life.
Darth Plagueis is this for Palpatine, Dooku, and Nute Gunray. Though unlike the other two, Palpatine was evil from the beginning, and the book merely shows how he became a Sith.
Warriors: The Rise of Scourge. It turns out that Scourge was, at first, just a cute little kitten with a crappy childhood. Desperate to impress the world around him, he is driven to first scare a dog away, then eventually actually kill a cat to maintain his peers' respect, which he claims his Moral Event Horizon.
The Doctor Who Missing Adventures novel The Dark Path tells how the Second Doctor sees his old friend Koschei become the Master, due to Love Makes You Evil. This may or may not be compatable with the Start of Darkness shown in "The Sound of Drums".
Fëanor of The Silmarillion has his over the course of several years due to a combination of manipulation, jealousy and anger issues, but the single defining moment is when he lets his anger and jealousy for his brother Fingolfin get the better of him and puts his sword to Fingolfin's chest and threatens to kill him.
Clariel: The Lost Abhorsen, forthcoming prequel to the Old Kingdom trilogy, is set to depict how its title character became Chlorr of the Mask, an evil necromancer who served as one of the main villains of the second and third books.
Barney in the first season episode "Game Night" of How I Met Your Mother. We learn how he evolved from a long-haired hippie guy into the Barnacle.
The Master gets a some of this in the Doctor Who episode "The Sound of Drums". It's revealed he was driven insane during a procedure on Gallifrey of staring into the Untempered Schism, a gap in the fabric of reality which exposes the time vortex. This then expanded upon in "The End of Time", where it turns out Rassilon, Lord President of Gallifrey during the last days of the Time War, retroactively drove the Master insane on his own command to establish him as a link between Gallifrey inside the time-locked Time War and the tangible universe outside it.
The episode "Six Months Ago" shows how Sylar first killed a man and stole his power. Two seasons later, another flashback from the same moment in Villains expands the story and shows Elle and Noah Bennet are at least partially responsible for him becoming a serial killer. Especially since Elle stopped Sylar from committing suicide by hanging in the first place.
The Volume 2 story arcs in feudal Japan show the Start of Darkness for AdamMonroe, and some arcs in the axillary graphic novels show the background of characters like Thompson and Linderman.
In Volume 4, we also get the background for Angela Petrelli as well as the beginnings of The Company.
Anya got one in season 7, although it's revealed her base personality hasn't changed as much as you'd think.
The books and comic books have reenacted Drusilla's Start Of Darkness at least twice. Since it involved Angelus murdering her family and torturing/raping her until she lost her mind, this is usually done to elaborate on Angelus' guilt.
Drusilla got a flashback in "Becoming Part 2" as well as the episode "Dear Boy" on Angel showing her as a human the time Angel started stalking her.
While Angel gets plenty of flashbacks on his spin-off series, the season 1 episode "The Prodigal" specifically shows his human life and eventual siring from Darla. Speaking of Darla, she gets an episode like this to herself in season 2.
The Cold Case episode "The Woods" explores the background of a Serial Killer who made his debut earlier in the season.
The Criminal Minds episode "No Way Out Part II: The Evilution of Frank" has the team delve into the past of Frank, who had appeared in a previous episode and was described by Gideon as "the most prolific serial killer ever".
The Season 3 episode "The Man Behind the Curtain" shows how Benjamin Linus initiated the Purge and became the leader of the Hostiles/Others, though he's more of an anti-hero than a villain.
One of the last episodes explains why the Man in Black is evil in a full-episode flashback (Jacob pushed him into the heart of the island and he popped out as a smoke monster).
Holocaust has this for a major plot thread with Erik Dorff, initially a man of conscience who joins the SS at the urging of his opportunistic wife, is ordered to oversee a Nazi death camp and eventually slides into becoming a monster that only Heinrich Himmler could admire.
Teen Wolf has this with "Fury" for Matt, "Visionary" for Deucalion, and "The Overlooked" for The Darach, as well as season one for Peter Hale.
The episode "Katerina" of The Vampire Diaries delves into the history of the eponymous Magnificent Bitch.
There were also episode detailing Callisto's rise.
Each of the Big Bads on Warehouse 13 get an episode (or details spread out over several episodes) explaining how they became the villains we know them as:
MacPherson used the Phoenix artifact to save his lover; by "dying" temporarily, he saw the afterlife, which from his point of view was nothing but darkness. He assumed this meant that there was nothing after life, and that all that matters is the now.
H. G. Wells lost her daughter, and started seeing only the worst in people; when she was de-Bronzed in the present, she saw things had only gotten worse and was pushed straight into Omnicidal Maniac territory.
Walter Sykes was corrupted as a child by an artifact that let him walk; when it was confiscated by Warehouse agents (specifically, Pete's mom), he became obsessed with getting it back and getting revenge on the Warehouse for taking it away.
Season 4 turns out to be one long SOD for Artie as his Enemy Within (manifesting as a hallucination of Brother Adrian) slowly consumes him.
Rumplestiltskin in Once Upon a Time: In order to prevent his son to be drafted into the ogre war, he gained magic powers by killing the Dark One.
"The Stable Boy" does this for Regina. Her ambitious and cold-blooded mother wanted her to marry up, but she was in love with a stable boy. When Snow White, then an innocent child, tried to help Regina by letting the ambitious mama know her stepmother-to-be was with the stable boy... Well, let's just say someone's True Love ended up dead, and Snow White ended up on the wrong end of a vendetta.
"The Miller's Daughter" showed how Cora became who she was. When she was a young woman, she was tripped by an immature Eva (Snow White's mother before she married Snow's father) who claimed Cora hurt her. The King of the land forced Cora to apologize on her knees or he wouldn't pay her for the flour. She would later use the emotions she felt here to channel her magic to spin gold.
Smallville is this for Lex Luthor. To be more accurate, season five, Lexmas. By Season 7 he's become the megalomaniac we all know.
Morgana Pendragon gets a similar treatment in Merlin though in terms of personality, she's more Magneto than Lex Luthor. By Season Three, she's ruthless and cold, by Season Four, she shows remorse, and becomes more human, though still a Well-Intentioned Extremist.
Well, the above is probably the intent of the series, realistically all the According To The Writer and the difficulty of turning a one-scene villain from the myth into a six-season character that had to survive to the end, it plays more like she's just schizophrenic and her evil personality turns up whenever a coin comes up tails somewhere in the kingdom. Her intentions would have to be coherent for her to be a well-intentioned anything.
Mordred counts as well. He's been in a total of three episodes although he is returning for series 5, and he's already got a Freudian Excuse, budding sociopathic tendencies, and become way too willing to kill people for a child who grew up with pacifists and can't be more than 9.
Season 1 of Carnivŕle delves heavily into Brother Justin's Start Of Darkness,as he develops from a compassionate and deeply moral laypriest into (at the very least) an unrepentant demon. It doesn't help that this was his bloodline and his sister murdered a bunch of orphans to engineer a personal tragedy for him.
For Carl Elias, it was his Mafia boss father killing his mother, and then trying to kill Elias.
For Samantha Groves (super-hacker "Root"), it was when her childhood friend was murdered and an adult she respected covered up for the killer. One of her first forays into hacking was done to visit Laser-Guided Karma on the killer.
The BIONICLE web-serial Mutran Chronicles and a scene from the book Swamp of Secrets reveal just why the formerly benevolent Brotherhood of Makuta turned against the Matoran Universe — it was because the peoples of the universe all attributed their efforts to preserve the balance of things to Mata Nui, and shunned them for being affiliated with the element of shadow. They got fed up with this. The comic Rise and Fall of the Skrall also details why the titular race wanted to overrun the desert region of Bara Magna along with its locals — they were driven out of their home-realm by robot assassins, and needed the space to fight back. Although it's to be assumed that being mean has always been their way.
There have been several in-character ones over the years. It can be argued that Randy Orton's slow descent from the suave Legend Killer to the sadistic Viper was a result of his ousting from Evolution, something he never got over. Evolution played a major part in Randy's character and actions, being the reason for his hatred for Triple H, extending all the way to 2009, four years after the stable had fully dissolved, and yet at the same time a blueprint for Randy's own power stable Legacy. It's also pointed to as the main reason why Randy had pretty much gone kayfabe-insane in 2009.
The New World of Darkness games all have a mechanic that encourages this trope for players: as the players sin or make immoral or counterproductive choices, they "degenerate", making future choices of slightly greater depravity and further degeneration easier and more likely. Each degeneration can also give a character a minor derangement such as a phobia or narcissistic tendencies, which can force further sins even if the player doesn't want to do so.
Fits this trope because at the bottom of the scale the player becomes so irrationally self-centered and evil that they cannot be played and become an NPC. What kind of monster depends on the game: for mortals, you become a Serial Killer, werewolves become a movie-style wolf-man that hunts humans for fun, changelings become completely unable to tell imagination from reality, vampires become ravening blood-crazed beasts, and mages turn everything in their area of effect into a Cthulhu Mythos story.
Super Robot Wars Original Generation Gaiden features the Start of Darkness of one of MX's Big Bad Albero Esto. It was only mentioned several times in MX, but in OG Gaiden, it's... rather full-blown and detailed.
In Alpha Gaiden, Char's Face-Heel Turn (since Char's Counterattack is part of Alpha 2) makes even more sense then it's UC Gundam verse reasons. During the course of the plot, he watches humanity keeping picking the Too Dumb to Live options and the horrifying results, and this disillusions and pisses him off enough to decide a Colony Drop is actually a good idea.
Sengoku Basara 2 Heroes features a Gaiden story for Maeda Keiji, which in fact was a Start of Darkness for a villain in the actual game, Toyotomi Hideyoshi. Hideyoshi starts out as a normal man, joining Keiji in pranks here and there, until he encounters Matsunaga Hisahide, who proceeds to humiliate him, and this causes him to become drunk with power, leaving Keiji and doing a lot of atrocities later on.
The Mass Effect novel Revelation serves as a Start of Darkness for Saren, showing how he came to be the villain in the videogame....though in the novel itself, he's already an extremeKnight Templar.
The comic Mass Effect: Evolution is one for The Illusive Man and Cerberus.
About one third of Crisis Core is spent on telling us how Sephiroth became the Magnificent Bastard and One-Winged Angel he is in Final Fantasy VII. Including remaking Cloud's flashback scenes from the original game to the actual moment he snapped, also a Start Of Darkness in their own right. This took advantage of advances in animation and allowed for Cloud's serious amnesia problem. (Also the Epileptic Trees spawned when he had dialogue right for a scene he was not actually present for. Some fans believe his delusion of Zack-ness was largely caused by some kind of Hojo-induced memory transfer.)
The game proved that he didn't suffer a sudden sanity loss, but rather more of a long line of betrayals that started as early as Gast's death and ended with the very facts of his own existence, which overall is both more believable (even allowing for Jenova's clear ability to mess with his brain) and more tragic. Due to Crisis Core we know that he had lost absolutely everything by the time this happened.
His Start Of Darkness could be pinned at conception, at the moment he broke, or just described as 'his whole damn life.' The man was born one of Hojo's experiments. Their suffering has been unquestioned more or less since Hojo was introduced. Though he still had just enough innocence left during that fateful practice duel that events afterward could almost be labeled 'Break the Cutie'.
Prince Luca Blight from Suikoden II had his after his family was ambushed by bandits in one of their official travels. He and his father managed to escape, but his mother wasn't so lucky. Luca wanted to go back for her but his father said that he couldn't risk his life as the King. His mother was rescued shortly after, but she didn't return to the castle. Several months later, Luca managed to find out where she was and quickly escaped from the castle in order to meet her. He had a arduous journey, and a man took his horse in exchange for a pear and some water (Luca didn't want to eat the wild berries or drink the muddy water he found in the way). Upon finally meeting his mother, she started to scream and rip her hair out when Luca came close. One of their aids explained that she was brutally raped by the bandits and got pregnant, as well as going completely insane and unable to stand near men. Luca then snapped. He went back to the castle, but not before meeting the man who took his horse, who he murdered with joy. He was also lucky enough to find a small village along the way, and found that the bandits, as well as his mother's rapist, were sheltered in a house there. He stormed inside the hut and caught everyone off-guard, murdering everyone. When he found the rapist, he tortured and killed him gruesomely. When he returned to the castle, two things were permanently scarred on his mind: the disgust for people and the pleasure he got when killing. Thus, Luca Blight, who causes a war for the sole reason of having an excuse to kill people by the thousands.
Heck, The Legend of Zelda: Skyward Sword serves to show a Start of Darkness for the entire series, seeing as every figure of evil that appears in the Zelda universe, Ganon or otherwise, is a result of Demise's curse incarnate.
Kagetsu Tohya fleshes out Michael Roa Valdamjong's start of darkness with the Crimson Moon and Drinking, Dreaming Moon scenarios. Roa actually wanted to achieve immortality because he fell in love with Arcueid and wanted to be with her for eternity, but mistook that feeling for hatred and thought he wanted to torment her for eternity.
Araya wants to take over Ryougi's body because he wants to reach Akasha, so all the meaningless deaths would at least have a reason and cease to be meaningless. ect.
In Live A Live, the hidden Medieval Chapter explains just who and what "Odio" is, and what made him into the demon of hatred influencing the various end bosses. It starts the young, heroic, Knight in Shining Armor Oersted on a fairly formula "Save the princess from a demon" plot, but... well, to say it all goes to hell about halfway through would be an understatement. By chapter's end, everyone in the kingdom believes Oersted to be the real demon (over something that was an accident, not that they saw that part of it), everyone who believed in him is either dead, or being "protected" from him, his best friend is revealed to be the one who ruined his life, and the princess declares her own hatred for him after he duels said traitorous friend to the death, and then commits suicide. Oersted is left with absolutely nothing. He then proceeds to just plain snap, and decides that if the world is so insistent that he is a demon, then a demon is exactly what he will be.
Dragon Fable's Fire War is a Start Of Darkness for Drakonnan. Once Yulgar's apprentice, Konnan called upon the hero to stop the rampage of the red dragon Akriloth upon his home village. But despite the hero's best efforts and the powers of the hero's recently acquired dragon, they were no match for Akriloth and the power of the Fire Orb. Akriloth, instead of finishing the hero, sadistically made them watch as he burned the village to the ground, killing Konnan's family. Konnan blamed the hero for failing to save them and when the evil pyromancer Xan got his hooks into him, this would turn into a desire for revenge that would soon threaten the entire land of Lore.
In the 1st Degree has James Tobin charged with grand theft and murder. There are indications in the interrogation tapes that the chain of events leading up to the events in the game go back to when Tobin and his wife Helen divorced. He apparently took it hard and just went on a steady downward spiral after that.
Kingdom Hearts Dream Drop Distance gives us the Start Of Darkness for series Big Bad Xehanort. What drove this man down his path of villainy? His Heartless went back in time and showed his younger self all of the evil acts he would do, even bringing him to the future to help them out. Even though he would forget what he saw and do, the actions he would take are now engraved in his heart. Xehanort isn't just happy messing around with the heroes- even his youth is fair game!
In Injustice: Gods Among Us, we learn that Superman starts his takeover of the world after the Joker tricks Supes into kill Lois and his unborn child, leading to the hidden Dead Man's Switch in her to activate a nuke and blow up Metropolis.
In Ravenmark Scourge Of Estellion, Livia Cassianus finds out that the late Emperor Sergius Corvius has always intended for her to inherit the Obsidian Perch (i.e. the throne of the Empire of Estellion). Before that, Livia falls in love with young Calius Septim. In the final cutscene, Calius gives his life to save Livia from the Big Bad pulling a Taking You with Me. Livia herself is scarred by the flamesoul explosion. At Calius's grave, Livia vows to remake the Empire into something new, claiming that traitors must be rooted out, and the blood of the fallen must be repaid. The description of the sequel Ravenmark: Mercenaries claims that six years have passed since then, and that the land is still reeling from the rule of the "Scarlet Empress".
Fate/Zero is mostly a retelling of the Fourth Grail War, making it the prequel of Fate/stay night. In it, Kotomine is still more or less a good guy, though all his mental issues are still present. While the war is going on and Servants are going down, Gilgamesh is needling Kotomine towards realizing what he is and descending into villainy.
Shion gets one herself during the Eye Opening chapter (takes about 2 episodes in the anime, culminating with the "distinguishment" incident). The events happen in every arc (having taken place one year before the story begins, but whether the events "detonate" depends on the arc.
The comic's author and illustrator, Rich Burlew, said in the Introduction that the greatest challenge of Start of Darkness was to tell Xykon's backstory without making him even slightly sympathetic. He solves this problem by making Xykon's every appearance push him farther beyond the Moral Event Horizon.
Burlew: ... [Xykon]'s completely and wholly unapologetically evil, but more to the point, he's kind of a dick.
The very first page of the book might have been teasing at it: Xykon is shown as someone who might come off as sympathetic for the first three or so panels, but revealed to be already evil before the page is over — at the age of four. And yet, Burlew does give him one simple human, if not quite redeeming, quality that makes his final descent, if not sympathetic, at least understandable. After being turned into an undead creature, he loses his ability to enjoy simple pleasures such as the taste of coffee. And then brutally murders the diner waitress because of it.
Burlew makes a point of not giving away Belkar's backstory in On the Origins of PCs (and the Belkar backstory comic for Kickstarter donors) for similar reasons, wanting the character to remain completely, unapologetically and unmistakably evil. Although, also to keep him funny. Evil isn't funny when it has a tragic backstory, just pitiable. Belkar does end up revealing a sob story in his childhood in the main comic... Entirely made up, it turns out, spun in order to gain roleplaying XP.
In Act 6 of Homestuck we learn Lord English's origin. He was a boy called Caliborn who dropped himself in a "dead" session of Sburb (the Absurdly High-Stakes Game the characters are playing) than slowly worked to become more and more powerful until he ultimately became a full-blown Eldritch Abomination. Also subverted in that we learn he was a scumbag even before he became Lord English. If anything he simply transformed himself from a Harmless Villain into The Juggernaut.
The Spoony Experiment spoofs this in its April Fools Day review of the original Final Fantasy. The Spoony One was driven insane trying to comprehend the game's time travel plot and became determined to invent his own method of time travel to stop the series from being made, ultimately causing his own time paradox by his future self appearing and presenting him with the technology fully formed. Along with a rather neat bit of acting, with Noah making a seamless transition from Spoony to Insano before our eyes.
Super Mario Bros. Z features a prologue where Metal Sonic becomes the deadly Mecha Sonic... and proceeds to destroy all of Mobius and kill all of Sonic's friends.
Interlude 19 of Worm follows Emma Barnes transition from an ordinary high school student to a ruthless bully determined to destroy Taylor's life.
The first eight arcs of the story can be considered one for Taylor.
The Dr. Robotnik of Adventures of Sonic the Hedgehog has his origin told in the episode "Best Hedgehog" as a high school student expelled because he tried to kill a romantic rival with a robotic snake to woo a girl he liked. He made sure that rival was his first prisoner.
The episode "Princess Cookie" was an episode all about this. Baby-Snaps the cookie is seen taking hostages and trying to steal the crown of Princess Bubblegum. Then, we learn of his past: he was an orphan living in a terrible orphanage, and one day, Princess Bubblegum showed up to read to the kids and cheer them up. The princess put Baby-Snaps on her lap and asked him what he wanted to be when he grew up. He jollily said: "I want to be a princess like YOU!" And the princess giggled innocently. That was the start of darkness for poor Baby-Snaps, who went on to threaten people with violence, take hostages, and attempt suicide, and end up in a mental hospital.
Avatar: The Last Airbender has one of these. "The Storm", aside from telling how Aang ended up frozen in an iceberg, gives us how Zuko got his scar and banishment; He spoke out against a general who believed We Have Reserves, but because it wasn't his place to speak out, he had shown disrespect, and the Fire Lord declared a duel to resolve the matter. Zuko accepted, thinking he'd fight the general, but because it happened in the Fire Lord's war room, it was the Fire Lord he'd disrespected. Zuko couldn't fight his father, and begged forgiveness. Instead, he got a fireball to the face and an exile that would not be lifted until he found the Avatar, considered a Snipe Hunt at the time.
The Batman's episode "Riddler's Revenge", depicting Edward Nygma's gradual transformation from geeky, unappreciated kid to murderous costumed criminal.
There is such an episode with the Riddler, wherein he creates a computer game that makes millions but is then fired by his publisher so that, as according to his contract with them, he doesn't get anything from it. And so becomes the Riddler in his attempt to punish his old boss. Notable in that this is one of the few episodes where the Villain of the Week escapes, unharmed and untouched. He was never there to be caught. At the ending, where the wealthy businessman is headed to bed. Shaking in terror all the way, he locks the multiple locks on his door, climbs into bed with a double-barreled shotgun, and cringes in paranoia.
The show is also credited with Mr. Freeze's backstory, who up to that point was just a cold-themed villain who turned up occasionally. The cartoon turned him into an Anti-VillainWoobie.
Temple Fugate, a Schedule Fanatic, decided to take a break from that schedule at the suggestion of then-Counselor Hill and things went horribly wrong, costing him a court case. Temple snapped and vowed revenge on Hill, now Gotham's mayor, becoming the episode's titular villain, the Clock King. He orchestrates a smear campaign against Mayor Hill as a prelude to kidnapping him and putting him in a Death Trap.
Temple Fugate’s case is a bit of a Deconstructive Parody of this trope, Temple had No Social Skills and was a Bad Boss long before the fateful day he took Hill’s advice. Even when confronted with the fact that Hill meant him no harm, Temple reconstructs this trope when he refuses to admit he's taken his obsession too far, being a Schedule Fanatic, being punctual is the only thing he cares for.
Batman:Give it up, Fugate! Hill committed no crime against you!
The tie-in comic book for the series described (but did not show) one for Arnold Wesker, a.k.a. The Ventriloquist. The Ventriloquist tells Batman that he was actually born into a mob family and was encouraged by his mother not to allow himself to drift into a life of crime like the rest of the family had done. Arnold promised her he'd do this...but then his mother was killed one night by a bullet meant for his father, and Arnold felt that he had to avenge her. Even after becoming a gangster, he still kept a framed photo of his mother on his person at all times - until Scarface, in a fit of jealousy (because the Ventriloquist had been keeping company with other puppets in an attempt to distance himself from Scarface), tore up the photo, causing Arnold Wesker's mind to snap for good.
The Boondocks episode "The Color Ruckus" shows the events in the life of Uncle Ruckus (no relation) that lead him to becoming a Boomerang Bigot.
The main purpose of the four-part Gargoyles episode "City of Stone" is to provide a framing story for one of these for Demona and Macbeth.
Skeletor got one in He-Man and the Masters of the Universe (2002). Turns out he was a powerful wizard trained by Hordak who wasen't completely bad (or at least not crazy). Then his face got flash fried off. After seeing that he has a skull for a face now, he completely lost it. The scene where his sanity finally snaps can scare someone as he starts laughing into the sky like the god damned Joker.
Powerpuff Girls: Mojo Jojo's origin was intinally revealed in the episode "Mr. Mojo's Rising". Mojo's telling of the origin paints him as someone who might come off as sympathetic but Utonium reveals that Mojo was just as unsympathetic when he was Jojo. In fact, Mojo may actually be more likable and sympathetic as a supervillain than he ever was when he was just a normal monkey.
While Samurai Jack's first episode explains the basic premise behind Aku, the episode "The Birth of Evil" tells his exact origins as a castaway shred of an Ultimate Evil which the Gods fought among the stars.
The Transformers Animated episode "Along Came A Spider", where we find that Blackarachnia's hatred against Autobots began when Optimus Prime and his friend Sentinel accidentally left her behind on an alien planet inhabited by spiders. While still inside the caves, Blackarachnia is accidentally mutated into a technorganic spider causing her to join the Decepticons as revenge for Optimus and Sentinel's betrayal.
The Venture Bros. offers a Start of Darkness for Phantom Limb in "The Invisible Hand of Fate." At one time he was a Mad Scientist in the dotty/well meaning sense, and chivalrous enough to turn down future-Mrs-the-Monarch's sex-for-grades proposition. Long story short, both he and Billy Quizboy were victims in a Gambit Pileup.
Nerissa of W.I.T.C.H. gets one for both her comic and cartoon selves, both the same: a leader of her generation's Guardians, the Oracle feared she was becoming too attached to the Heart of Candracar and gave it to her friend Cassidy. When Nerissa demanded it back and Cassidy refused, Nerissa slew her in misguided rage.
"The Last Tough Customer" on Arthur shows Molly's in The Teaser: when she was in maybe kindergarten or first grade, a couple of older kids teased her about her poufy hair. She took out her hairbands, letting her hair fall across her eyes, and became a bully.
The Wakfu special "The Legend of Goultard" reveals how Goultard became a demonic berserker in the game. He was originally a brave and mighty hero who had a good life with a wife and three children. Then a villain named Katar kidnapped them to lure Goultard into a fight. Goultard tracks him down and is horrified when he discovers that Katar has already murdered his entire family. After a brutal No-Holds-Barred Beatdown, the defeated Katar explains that he did this to free himself from the demon inside him by presenting it with a tastier target. The demon is drawn to Goultard's hate and fury and possesses him. Goultard finishes off Katar and eventually becomes an immortal demonic warrior who rules over a realm of madness and horror. Though in the series proper, he is eventually freed of the demon but retains the immortality and becomes Sadlygrove's mentor.