Origin Story

Much like how ordinary good guys eventually became heroes, villains themselves also have an Origin Story of how they eventually become the villains that the heroes ultimately face later on. This is called the Start of Darkness.

In order to keep their characters current, various media companies frequently rewrite the origins of their oldest characters; this goes from adding details that do not contradict earlier facts to a totally new origin which makes it seem that it is an altogether different character.

"Origin story", or pourquoi story, is also a term used in mythology. It can refer to narratives of how the world began, how creatures and plants came into existence, and why certain things in the cosmos have certain qualities.

How the characters eventually became the very villains that everyone known were varies, though some cases tend to be related with Mad Science.

Variations of Origin Story Scenarios
Each villains had various scenarios regarding how they went to the dark side, which includes:
 * Being Tortured Makes You Evil Scenario: Popularized by TV Trope of the same name, this scenario revolves around how a villain comes to being as product of torture or abuse, mentally and psychically. Motivation of torture that resulting a character become evil in this way ranges from simply mere amusement to more complex and systematic one, the way a Big Bad indoctrinated his subjects.
 * Rival Turned Evil: Also popularized by TV Trope of the same name, thsi scenario revolves around how a villain comes to being as mere rival of the hero until they went bad.

Anime and Manga

 * Naruto:
 * Orochimaru: As result of his parent's death during the war, Orochimaru became disillusioned with the reality where all living being would die someday, and sought to become the strongest shinobi in the world and an immortal via combining kinjutsu and various experimentations.
 * Mizuki: Due to his personal obsession to be the strongest shinobi that escalated to unhealthy level, Mizuki became indifferent with by things like morals or teamwork (traits which made Orochimaru became interested by him that he gave him Curse Mark that would activated through certain drug combination as seen in anime) and ultimately became a villain and one of Naruto's most dangerous advesary during his youth.
 * Dragon Balls series:
 * King Piccolo: The demonic Piccolo was originally the evil part within a powerful Namekian. When the said Namekian expelled all evil within him, his expelled evil manifests into Piccolo whom born with flesh and blood like regular Namekian, but only carry malevolence within his creator. In spite of his, his final spawn, Piccolo. Jr., whom Piccolo created to avenge his death and initially believed as evil as his parent, revealed to be born as regular Namekian who lacked his predecessor's malevolence in spite of possessing powers that as immense as the former. The lack of his parent's malevolence of course, what contribute to Piccolo Jr.'s choice to become a good person.
 * Scar from Fullmetal Alchemist: Sought revenge on every State Alchemist for the Ishvalan massacre many of them have comitted.

Comic Books

 * Man-Bat: Dr. Langstrom's attempt to cure his deafness with help of serum based on bats' DNA and to help those whom are deaf and blind and test the serum on himself, only to ended up turned into humanoid bat-like monster. This prompt Batman to cured him to normal.
 * The Lizard: Similar with Dr. Langstrom's case, Dr. Connors creates a serum based on lizards' DNA to create regenerative serum that would beneficially help those whom became disable or severe injuries where he test the serum on himself that not only regrows his right arm, but ended up transformed into a humanoid lizard where Peter Parker had to create a cure to restore him.
 * Magneto: Having survived Nazi Holocaust during events of World War 2 where his powers manifests for the first time and sees mutants whom mistreated due to jealousy, hatred, or prejudice in reminiscent way Nazis mistreated many innocents includes himself, he sought to ensure that mutants would live in peace in the world, but in more extreme way than his friend Xavier's methods as he also believed that mutantkinds are better than humanity.
 * Scorpion: Mac Gargan, the nasty private investigator whom hired by J. Jonah Jameson to stalk on Peter Parker to find out how he can easily get his alter ego Spider-Man's photos, conspired with his client to defeat the web-slinger with Dr. Stillwell's help whom enhanced his DNA with that of scorpion that grant him superhuman capabilities and a scorpion suit and become the notorious supervillain called Scorpion upon the revelation of his genetic enhancement would render him insane as Dr. Stillwell's feared.
 * Apocalypse: When EN Sabah Nur was born, he was abandoned of having grey skin, blue lips, and unusual facial markings. He would eventually rise to become a strong warrior and soon, a powerful mutant by mastering his powers and acquiring alien technology. He would be worshipped as a god by many and years later, he would awaken from dormancy into the modern world and pose the biggest threat to the X-Men as he would seek to destroy the current world and build a new one in his image referred to simply as the Age of Apocalypse.
 * Jason Todd (nicknamed Red Hood): Disillusioned by Batman's code not to kill his enemies, which worsened with his near-death experience by Joker whom intended to killed him, Jason Todd, upon survived his ordeal (resurrected in some versions soon after killed outright), decided to become a villainous vigilante called Red Hood.
 * Darth Wredd: Darth Wredd himself was a normal human boy when his hometown was destroyed by a band of barbarians and was eventually inducted into the One Sith after his homeworld was invaded by the Galactic Empire lead by Darth Krayt. He was instructed in the ways of the dark side of the force, but soon found out that his entire homeworld was destroyed by the One Sith and defected to seek revenge against them and restore the Sith Order under the ancient Rule of Two.

Animated

 * Aggie Prenderghast from ParaNorman: Sought revenge on the townspeople as well as Judge Hopkins for their discrimination against her.
 * Lord Shen from Kung Fu Panda 2: He became disturbingly obsessed with fireworks as he saw they could also be used in warfare. His concerned parents consulted the Soothsayer who foretells Shen's destined downfall at the hands of "a warrior of black and white." Shen overhears this prophecy and decides to avert it by massacring a nearby village of  innocent pandas. Shen returns to his parents with sadistic joy, but his parents, horrified that their son would do such a wicked deed, banished him from the city as punishment. Shen, believing his parents hated and wronged him, vows revenge that he will return to Gongmen one day.
 * Syndrome:
 * Professor Screweyes: When he was a child, he was snoozing by a berry bush, and after a berry fell upon his eye, a crow pecked for the fruit, while at the same time, taking his eye. Ever since then, Screweyes decided that the world around him made no sense at all and therefore acts accordingly to life's senseless ways by using fear to get his way. Because of his incident, he had a fear of crows, and uses this as his act. This way, even while he fears the crows, they are still under his control.
 * Lots-O'-Huggin' Bear: Was the favorite toy to a little girl named Daisy. One day, Daisy's family took a little road trip, when as the family left, they accidentally left Lotso, Chuckles and Big Baby behind. Lotso and his allies walk for miles before finally reaching Daisy's house--only to discover Daisy has a new replacement Lotso. This tragic event causes Lotso to snap and turn evil and insane and he lies to the other toys that Daisy replaced them as well. Eventually, they came across Sunnyside Daycare and Lotso took control of the place and ran it like a prison for all the toys who resided there.
 * Francis E. Francis: Was a worker at Baby Corp known as the Super Colossal Big Fat Boss Baby eventually working himself up as its new CEO until he got kicked out for having lactose intolerance as he couldn't drink a secret formula that made babies act like adults, which resulted at him getting older, much to his dismay. After getting replaced and fired, Francis was sent to be adopted by the Francis family with an older brother named Eugene. Ever since then, Francis angrily vowed revenge on Baby Corp for this. Growing up to become a middle-aged businessman, Francis founded Puppy Co. to act as a rival to Baby Corp, becoming more popular to the public.
 * Robert Callaghan aka Yokai from Big Hero 6: Sought revenge on Alistair Krei for his role in his daughter's perceived death.
 * Phango: Was born with one blind eye and was abandoned by his clan as the runt of the litter. He was thrown in the magical water pond in the clan's lair. This only made him stronger giving him an extra, almost mystical strong sense of smell. He become ruthless killer after killed his clan in revenge. He is obsessed with being whole.

Live-Action

 * David Banner: Sought revenge on General Thunderbolt Ross for berating him for experimenting on himself and his son Bruce, and later tried to absorb his son's powers after gaining matter-absorbing powers.
 * Toho's Godzilla films & MonsterVerse film series:
 * Godzilla: Godzilla' Toho incarnations were originally dinosaur-like prehistoric intermediate reptilian lifeforms (clarified as Godzillasaurus in Heisei series and unspecified marine reptile in Godzilla: Resurgence) mutated by radioactive materials produced by either nuclear tests or nuclear waste dumped to his habitat which mutated him into nuclear-powered kaiju. MonsterVerse incarnation however, gave him more realistic origin as he was established as known still-living member of prehistoric reptile species that evolved by natural radiation on Earth during prehistoric era.
 * Destoroyah: Destoroyah are malevolent monster that originally result of prehistoric crustaceans empowered by chemicals within Oxygen Destroyer used to kill first Godzilla. But in IDW comics, he was instead products of chemicals (with presumably coincidentally identical properties with Oxygen Destroyer) used by Devonians to bio-engineered same prehistoric crustaceans with similar results.
 * Keizer Ghidorah: Keizer Ghidorah is cryptic variant of Ghidorah with cryptic origin, as it's unclear whether Xiliens either engineered him or merely found him before out him under their control.

Literature

 * Lord Voldemort: Birthed from the impure, loveless union between Tom Riddle Sr. and Merope Gaunt, and later sought for immortality via Horcruxes when he discovered his magical inheritance, Lord Voldemort sought to rule the world which is pure from humanity and became the Dark Lord that is infamous to all to the day.
 * Sauron: Formerly a Maia of Aulë the Smith named Mairon, Sauron was seduced into servtitude for Morgoth, and is reduced into a fearsome new Dark Lord.

Live-Action TV

 * Azazel (Supernatural): One of the Princes of Hell created by Lucifer, Azazel schemed to create many Special Children who could potentially serve as his master's vessel and released many demons, chief among them Lilith so she could begin breaking the 66 Seals of Lucifer's Cage. He fed various babies, including an infant Sam Winchester, his blood to give them psychic abilities; he also murdered Sam's mother Mary, leading Mary's husband John and their two sons to become Hunters.

Videogames

 * Alex Mercer: After the ill-attempt escape from Blackwatch soldier and is gunned to death not long after destroyed the vial of Blacklight virus that he took with him, Alex later rises as the new being which code named Zeus where the later plot twist revealed that Alex was already dead and his body actually possessed by the very virus that he released whom unknowingly took his identity.
 * Count Bleck from Super Paper Mario: Heartbroken at the disappearance of his beloved at the hands of his father, Lord Blumiere sought to destroy all the worlds for the world is meaningless for him without her.
 * Cole MacGrath: Cole was originally an ordinary man who worked as a courier and had his future self Kessler awakened his electrokinetic powers awakened with Ray Sphere explosion he instigated before faced several arranged tests as ordeal to see whether he will choose good path or evil path.

Web Series

 * Jeff the Killer: Jeff was originally normal young man with surprising agility that his brother Liu willing to take blame over his fight against a trio of bullies until the same bullies incinerate his face alive in their next fight. Unable to cope with his injuries, he went mad and reduced into serial killer.

Western Animation

 * Lord Garmadon: Lord Garmadon used to be a human until being bitten by the baby Great Devourer where the said snake's venom ultimately corrode his mind and turned him into a black armored-clad, four-armed evil entity. It's later revealed how Great Devourer's venom corrupt Garmadon's mind and turned him evil like that was due to corruption within the Devourer's venom contained part of more malevolent being called Overlord.
 * Blight: Prior to his transformation into Blight, Derek Powers was a corrupt businessman who killed his own employee Harry Tully in cold blood to test a nerve gas that his company invented and ordered his subordinate Mr. Fixx to murder Warren McGinnis who learned about the gas; this led to Warren's son, Terry, to become the new Batman and stop Powers' evil plan to sell the gas to Kasnia. However, in the ensuing fight, Derek ended up exposed to the nerve gas; to make matters worse, radiation treatment intended to save Derek's life mutated him into a monstrous creature with glowing transparent flesh and a black skeleton.