Because almost every character began their life as someone good, the moment where that changes is a powerful beat in the story. Typically this moment happens early on, to establish why there's a conflict in the first place, but it can also be established via Origins Episode or Flashback later in the story. This is the moment when a character who could have been on the side of Good (or at least not taken any actions) decides that the only way that they can get what they want is to become a villain.
This, naturally, is especially common with Fallen Heroes, who usually get a Downer Ending where they lose faith in themselves and/or humanity. This will be especially poignant if they Used to Be a Sweet Kid (see also: Freudian Excuse). The moment doesn't always mean that a heroic character became a villain, they just need to have had a moment in their lives, where they didn't ruin lives and haven't yet made choices that hurt others. If this is the subject of the main plot, you may be watching a Protagonist Journey to Villain or Big Bad Slippage. Details of a usually (but not always) Dark and Troubled Past may be revealed.
The Trope Namer is The Order of the Stick prequel Start of Darkness (overlapping with Origins Episode), whose title is itself referring to the 1899 Joseph Conrad novel Heart of Darkness, which tells the story of the protagonist's journey down the Congo river to rescue the mysterious Mr. Kurtz, an experience that changed his entire outlook on life for the worse.
- 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 transferred to his friend.
- Shown in a prologue in The Incredibles on how Mr. Incredible giving Buddy, his #1 fan, the cold shoulder eventually turned him into Syndrome.
- 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.
- The LEGO Movie 2: The Second Part: Rex Dangervest is revealed to be Emmet from a future timeline in which he was accidentally thrown under the dryer and forgotten by Finn, causing him to become bitter and cynical.
- 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!
- Randall in Monsters University starts to despise Sulley after Sulley accidentally humiliates Randall in the last round of the Scare Games.
- Toy Story 3 gives us a flashback of Lotso-Huggin' Bear being accidentally abandoned in a field on a picnic, then replaced with an identical model. The flashback's narrator tells us "Something changed that day inside Lotso. Something snapped." That's when Lotso lost all trust in humans and started his path on the dark side, eventually becoming the evil ruler of the Sunnyside Daycare Center.
- 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.
- Disney Live-Action Remakes:
- Maleficent has this happen to both the titular character and Stefan. Maleficent's is Stefan betraying her trust and cutting off her wings while Stefan's is more subtle: initially it was his ambition to become king that led him to betray Maleficent in the first place, but Maleficent vengefully cursing Aurora drove him into even further villainy and madness whilst trying to defy it.
- Cruella follow the eponymous character from a young grifter Waiting for a Break in the ruthless world of fashion to a fur-obsessed criminal.
- Dracula Untold explores the origin story of the man who became the legendary vampire, Count Dracula.
- Freddy's Dead: The Final Nightmare has shades of this in the flashbacks of Freddy's memories when Maggie goes inside his head.
- Ghosted (2023): Leveque, a former French secret agent, relates to Cole that nearly dying near Kabul made him realize how he'd given all he had to his country without gaining anything for himself. The realization convinced him to become a criminal, selling weapons of mass destruction on the black market for profit. Cole, pretending he's a CIA agent, claims later that Leveque's story inspired his own turn to crime and selling Aztec (a WMD).
- 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.
- James Bond:
- Francisco Scaramanga of The Man with the Golden Gun tells his to Bond while watching a kickboxing match.
Scaramanga: 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.
Janus: 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.
- Francisco Scaramanga of The Man with the Golden Gun tells his to Bond while watching a kickboxing match.
- Joker (2019) details, as the title suggests, the origins of it version of the Joker, a man named Arthur Fleck who suffers from mental illness who's picked on by society and snaps after getting his stand-up mocked on National TV and learning his mother lied about him being Thomas Wayne's son, learned he was really adopted, and that his adopted mother let him be abused.
- A Murder of Crows: Corvus' had occurred when the hit and run driver who killed his family got Off on a Technicality. He saw that the man himself was remorseful, but his lawyer simply delighted in winning (and in his pay of course). So he became Corvus' first victim, and other amoral attorneys followed.
- Saw has Jigsaw's origin story in Saw II before it's expanded on in Saw IV. Saw II is less of a sympathy play as while it gives a perfectly logical reason for Jigsaw's actions, it's still not rational enough to be sympathetic. Saw IV is a more conventional bid for sympathy, but it could also be viewed as simply setting up his state of mind for Saw II.
- The 1951 Scrooge explores this during the Christmas Past sequence, more so than other adaptations of A Christmas Carol.
- Split actually counts as one, when you take into account the film is set within the same universe as Unbreakable, thereby making the film a Super Villain Origin Story instead of a Super Hero one.
- The Star Wars Prequel Trilogy is essentially a Start of Darkness for the whole franchise, showing how the Empire came to be and how Anakin transformed from an idealistic young Jedi and the prophesied Chosen One into the Emperor's brutal, mutilated enforcer Darth Vader. The films (and later Star Wars: The Clone Wars) show that his Fatal Flaw is ultimately that he cares too much about those close to him and is willing to get very violent very fast to protect them, even if it means destroying the Republic and the Jedi Order if it can earn him a chance to save his wife from potentially dying in childbirth, which makes him all the more vulnerable to Palpatine's corruption. His journey to the Dark Side truly begins in Attack of the Clones when he starts having visions of his mother Shmi in pain and goes to Tatooine to find her. He did... mere moments before she dies in his arms after who knows what kind of abuse at the hands of the Tusken Raiders who had captured her. Anakin then murdered every last living thing in that village—men, women, children, and animals—and burned it to the ground. He doesn't get his iconic black life support suit until the end of the next movie, but it's safe to say that this is when Darth Vader was born.
- The Texas Chainsaw Massacre: The Beginning is this for Leatherface.
- X-Men: First Class:
- The film depicts Mystique as an insecure young woman looking for a purpose... and she finds it. Taken even further in X-Men: Days of Future Past, where she effectively becomes a Dark Action Girl. Xavier even states that her first deliberate murder of Bolivar Trask in the original timeline "is when Raven became Mystique."
- The film is also one to Magneto. In the prologue, we see his experiences living in a Nazi concentration camp, where he witnesses his mother's execution. At first, he and Xavier are Fire-Forged Friends, but Magneto slips more and more into villainy until he declares a new faction of the X-Men, whose objective is not to hide their mutation among humans. This would eventually descend into the supremacist faction of the original films.
- Doctor Steel's Lament for a Toy Factory.
- Gloryhammer's Big Bad Sorcerous Overlord Zargothrax finally gets on in "The Keeper of the Celestial Flame of Abernethy", although they keep it simple:
Travel back 1000 years to a time before Dundee
There lived a simple peasant weaving baskets in a field
But when a prince destroyed his home to build a kingdom new
He forged a pact with chaos, swore to bring him to his doom.
- 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 gone kayfabe-insane in 2009. The event had such an effect on him that one could argue that it defines his entire career. Only recently has he managed to get over it.
- Half-jokingly, fans said that Ultimate Warrior's No-Sell to Triple H's Pedigree at WrestleMania 12 was the moment that Triple H decided that he will use his burial shovel to bury those that stand on his way to reach the top of the wrestling world.
- Matt Hardy was never the same after his long-time girlfriend Lita cheated on him with his behind-the-scenes best friend Edge. There were a lot of moments beforehand that really pushed him close to the breaking point, but it was unquestionably this event that made sure there was no going back for him. What followed afterwards was a professional and personal breakdown that lasted several years, and even after getting his life back together, the emotional trauma had built up to levels that eventually culminated in the birth of "BROKEN" Matt Hardy.
- New World of Darkness:
- The 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. It 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 ( And become True Fae if they're powerful enough), vampires become ravening blood-crazed beasts, and mages turn everything in their area of effect into a Cthulhu Mythos story.
- In the fan game Genius: The Transgression, Genii have an alternate way of getting down to the bottom- failing Unmada checks. The first failure turns you into an Unmada, a Genius who has lost touch with real science and believes that his wonders are the true way the world works. Unmada aren't necessarily evil, but they are dangerous (One could, for example, see nothing wrong with Skynet's modus operandi), and if they fail a further Unmada check, they completely lose themselves to Inspiration and become Illuminated (completely alien and amoral intelligences), just the same as if they had fully bottomed out their Obligation. It's both easier and more common for a Genius to become Illuminated in this way.
- Magic: The Gathering: When Nicol Bolas was very young, he witnessed his sister Merrevia Sal be murdered by primitive human hunters. This seems to be the root of his deep-seated fear of death and his need to be more powerful than the humans.
- 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.
- Beast Wars: Uprising: The story "Identity Politics" is one for Scorponok, and Megatron. The former starts off as a senior worker at a refinery, and the latter a good-natured, approachable administrator who's even willing to talk with Maximals (a rarity in the stratified Cybertron). Then Megatron's boss shafts him to further his own political career, Scorponok makes a casual comment about finding a stash of energon of their own, and it all snowballs from there...
- 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.
- About one-third of the Matsuribayashi chapter of Higurashi: When They Cry is spent giving Nurse Takano one of these.
- Shion gets one herself during the Meakashi chapter (takes about 2 episodes in the anime, culminating with the "distinguishment" incident). The events are implied to happen in multiple arcs (having taken place one year before the story begins, but whether the events "detonate" depends on the arc) with clear exceptions like Saikoroshi-hen where Shion stayed at St Lucia to support her sister as much as possible from far away.
- Red vs. Blue:
- The Project Freelancer Saga is essentially this for Maine, and a downplayed version for Carolina and Washington (who become Anti-Heroes, but not completely evil). Past Maine was ruthless in combat, but still a loyal teammate who saved Wash, Carolina and York from a collapsing building. As the flashbacks continue, he goes mute and gets Sigma to compensate. Sigma becomes interested in metastability and brainwashed Maine to help him, eventually turning him into the Meta, arc villain of The Recollection. Carolina was Freelancer's top operative, but being constantly shown up by Tex wore on her and made her temper worse and worse. After finding out the full story behind Project Freelancer, she became bitter and driven to vengeance, uncaring of even her own team. Wash was the adorable Butt-Monkey of his team, but then he had Epsilon implanted, which gave him all the memories of the Director's crimes and a second-hand mental breakdown, leading to the cold and vengeful character who debuted in season 6.
- Season 15 has the Desert Gulch Chronicles flashbacks, which detail the time when the Blues and Reds were in the same situation as the Reds and Blues — playing Capture the Flag in a box canyon in the middle of nowhere. One member of the Blue Team, Mark Temple, was childhood friends with Biff, a member of the Red Team. They were even about to try to get Biff medically discharged so he could return to his girlfriend, but before they could do that, Agents Carolina and Texas were sent down for a battle. In the battle, Tex accidentally killed Biff. She and Carolina left the gulch and never thought of it again... but Temple had just lost his best friend, and the revelation that Biff had died for nothing more than a training exercise (and the UNSC had voluntarily given them up for it) broke him. He eventually started tracking down and killing Freelancers (even ones who had nothing to do with what happened) with the rest of the Blues and Reds, and plotted to destroy the UNSC.
- The Character Trailer for Volume 6 shows the trajectory of Adam Taurus, from his earliest days as a violent pro-Faunus activist to his actions during the show, being a villainous anti-human that partakes in genocides.
- The Volume 6 episode "The Lost Fable" reveals how Salem transformed from being an ordinary woman into Remnant's Ancient Evil. Imprisoned her entire life by her abusive father she was unable to cope with the death of Ozma, her saviour and lover. Punished with Complete Immortality for trying to trick the gods into resurrecting him, her vengeance destroys humanity, and she ends up corrupted after trying to kill herself in the divine Pools of Annihilation. After the gods resurrect Ozma to try and redeem the reborn humanity, the lovers clash over Salem's destructive urges, accidentally killing their four daughters and locking them into a bitter Forever War for the fate of humanity.
- The Volume 8 episode "Midnight" reveals how Cinder Fall became a sadistic villain. Adopted and raised by a cruel Atlesian hotelier and her daughters, Cinder is worked, starved and tortured as a child-slave. A local Huntsman sympathises with her, secretly training her to become a Huntress so she can escape when she comes of age. Unfortunately, the years of endless abuse eventually catch up to Cinder when she snaps and kills her adoptive family; once her mentor tries arresting her, she also kills him. Now she's obsessed with becoming strong, powerful and feared and treats others as her abusive step-mother once treated her.
- 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.
- In The Inexplicable Adventures of Bob!, Fructose Riboflavin has been the greatest criminal in the Nemesite Empire for the better part of two thousand years. But he Used to Be a Sweet Kid. Then his dad died...
- Mag Isa: The villains in this comic had miserable lives before they became villains.
- The Trope Namer — admittedly more recent than most of the other examples here, but it's still a pretty cool title — is Start of Darkness, prequel to The Order of the Stick detailing Evil Overlord Xykon and his Dragon Redcloak's past. Xykon, unlike most, is astonishingly evil from the get-go. We get to see him become more evil... but more importantly, a lot more competent. Redcloak is a whole other story.
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 XPnote . Word of God, as stated in On the Origins of PCs, is that this is to represent how some players create elaborate backstories for their characters complete with family and previous obligations and some... don't.
- 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 flashback chapters of Evil Plan show how Stanley turned from an idealistic inventor into Urbane City's first super villain. He found out his accountants were selling his inventions to super villains, he gained telekenesis via a self-experiment and accidentally killed his best friend trying to recreate it. The final scraps of his spirit crushed by trying to earn an honest buck as a bank teller, from there on he turned his company into a legitimate front, started building super gadgets, hired some minions and an engineer, opening the doors to the main story.
- In El Goonish Shive, Part 1 of the arc Sister 3 is Pandora's backstory showing how the premature death of her husband started her down the path toward With Great Power Comes Great Insanity.
- Belkinus Necrohunt: During the journey, the party gains occasional clues that Kara Miharian parted ways with Chandrelle and devoted herself to dark magic after their other sister, Abigail, was killed in the war eighty years ago.
- 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.
- The version two finale of Mega64 reveals how Dr. Poque became the Mad Scientist he is today.
- Dr. Horrible's Sing-Along Blog has several prequel comics which describe how and why Dr. Horrible decided to try to become a villain in the first place. The show itself is a Protagonist Journey to Villain.
- 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.
- From Killerbunnies, we have Genevieve, a sixty-nine-year-old bitter, manipulative, and foul tempered rabbit, who is a Black Widow, however, according to her backstory, her start of darkness came when she found out her first husband, Malcolm, was having an affair and, in her fit of upset, she poisoned him, leading for her continuous kill up to six husbands afterwards.
- Thanks to Ascended Fanon, Team Four Star's Let's Play of Dragon Ball Xenoverse is this - their character Dumplin, after ascending to the position of Demon God by killing Demigra and honing his powers, becomes/became Mr. Popo as he is in Dragon Ball Z Abridged.
Cell: Let me weave you the tale of my origins. Though I must preface it by saying it does drag on in places, so I'll try to cut down on the Filler. It all begins, as many stories do, with a girl shooting a young boy in the face...
- Happens in-universe in Dragon Ball Z Abridged, when a reporter asks Cell where he came from. Cell proceeds to recount the entire story of Dragon Ball up to that point (offscreen, thankfully).