From Nobody to Nightmare

Nobody to Nightmare is a similar is a villain's Start of Darkness that involves a  non -Badass normal taking numerous levels in badass, whether instantly or over the course of a story, thus becoming essentially nothing but a giant walking, talking bucket of villainy.

Through a series of unlikely coincidences, or through accidental or unknowing actions on the part of the heroes themselves, or as a result of things which should never have happened — and in most alternate timelines  don't  happen, but just barely by the skin of their teeth,  do happen — someone who might have stayed an insignificant nothing is transformed into a nightmare that grows and grows, absorbing the power to rend civilizations to dust and bring universes to their knees.

Animated Film

 * Lotso was once a soft-spoken, gentle, lovable teddy bear who first started out as a Christmas present for Daisy, a young kind-hearted girl who immediately became overjoyed the moment she unwrapped him. Things suddenly take a dramatic turn when on a family trip, as Daisy happily played with her toys including her very special Lotso, she fell asleep after lunch, accidently leaving her toys behind. Lotso and his friends decided to go back home afterwards, but by the time they got there, it turned out Daisy bought another Lotso to compensate for the original one she lost during the trip. Right when Lotso saw this,  something snapped inside him that day... It should be noted, that Daisy only bought another Lotso because he symbolized how much she truly loved her original. As pointed out by Woody, it was  Lotso  who abandoned Daisy to begin with.
 * Tighten: Hal Stewart started out as awkward cameraman with a crush on Roxanne Ritchi, but obtained superpowers via Megamind and became the supervillain Titan, an Omnicidal Maniac who is willing to obliterate an entire city just because Roxanne won't give into his even more disturbing advances.