Origin Story

Much like how ordinary good guys eventually became heroes, villains themselves also have origin story on how they eeventually become the villains that the heroes ultimately faced later on.

In order to keep their characters current, comic book companies, as well as cartoon companies, game companies, children's show companies, and toy 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 make it seem that it is an altogether different character.

"Origin story" or pourquoi story is also a term used in the study of myths. 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 villain that everyone known to the day were varies, and some cases tend to be related with Mad Science.

Anime and Manga

 * Orochimaru: As result of his parent's death during the war, Orochimaru become 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.

Video Games

 * 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.

Comic Books

 * Man-Bat: Dr. Langstrom's attempt to cure his deafness with help of serum based on bats' DNA 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 hands, but ended up transformed into humanoid lizard where Peter Parker had to create a cure to restore him.

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 infamous to the day.