Godzilla's origins vary based on the film, but they all agree that he was some sort of reptile on a small island near Japan that was hit by nuclear fallout and turned into a colossal, radioactive killing machine.
TRON: The MCP went from a simple chess program to a program designed to oversee the company's computer network and eventually plotted to take over the computer systems of both the Pentagon and the Kremlin.
Iron Man 3: Aldrich Killian started out as a sort of sad and nerdy scientist with a physical disability, but 13 years after being stood up by Tony Stark, he had become the ruthless and homicidal true alter-ego of The Mandarin, and the creator of the Extremis virus.
The Amazing Spider-Man 2: Maxwill Dillon was a nerdy electrician whom everybody picked on or ignored, who then deluded himself into thinking he was Spider-Man's sidekick after the hero saved him and showed him compassion. After gaining electrical powers, he decides he doesn't want to be ignored anymore, becoming a very powerful supervillain.
The A Nightmare on Elm Street sequel Freddy's Dead: The Final Nightmare attempts to humanize Freddy by showing him in flashbacks as a creepy young boy driven into bludgeoning a school hamster to death by taunting classmates, and then as a quiet, unassuming family man with an unhealthy obsession with serial killers, who actually becomes a serial killer himself for a vaguely sympathetic reason. That's all offered in contrast to the present day, where, after a Deal with the Devil made while he was dying, he's turned into a prophesied nightmare king who's left his hometown in ruins, its surviving inhabitants insane, and now has designs on the whole world. The disturbed but comparatively sympathetic man he used to be reappears briefly near the end of the film in what seems to be a Fighting from the Inside moment, but it's just as likely that it was a trick to briefly lull the heroes into letting their guard down.
Well, it's hard to imagine that Freddy was ever truly normal. Supposedly, he was born after his mother was raped by hundreds of inmates in an insane asylum (causing him to be called the "Son of a Thousand Maniacs" frequently. If there was any way that a child's conception could make him turn out bad, this would be the way.
In Star Wars, Anakin Skywalker started out as a slave on Tatooine. That's about as low as you can get in a galaxy far, far away. Through the will of the Force he was found and raised by the Jedi to be the Chosen One destined to destroy the Sith. Through the will of Palpatine he destroyed the Jedi first. And then he ruled the galaxy with an iron fist, as The Emperor's right hand man and destroyed an entire planet, culminating in him killing The Emperor with his own hands. Revenge of the Sith strongly hints that Anakin was the product of Darth Plagueis's twisted attempts at "creating life" through the power of the dark side, though the person claiming this is a Manipulative Bastard.
In Star Trek, Nero was just an ordinary captain of a mining ship when his home planet was destroyed and he got sucked into an alternate timeline. In the past, his mining ship can outfight any other vessel, and he has the capability to destroy planets and alter the timeline.
This applies to the Narada as well, at least according to The Countdown comic series. In its original time setting, it was a simple Romulan mining ship as Nero said. Later however, it's upgraded with reverse engineered Borg technology from the Tal Shiar, turning it into the starship equivalent of an Eldritch Abomination as well as one of the most powerful ships in Trek period (even in the original timeline).
There Will Be Blood. Though it occurs in a microcosm of a small mining community, the overarching theme of the whole film is this trope, happening in slow motion.
Happens to the nebbish protagonist of 976-EVIL, a film whose main claim to fame is that it was the first film directed by Robert Englund.
The Crow is a heroic example, where a murdered goth-rocker is resurrected as a superpowerful avenging angel.
Toy Story 3: 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. Along with her other two toys, Big Baby and Chuckles the Clown, she had so much fun playing with them, as she loved all three of them equally. But, according to Chuckles, Lotso was unique to her most of all. 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.
Randall Boggs started out as a nerdy college student before he fell in with a bad crowd. His Start of Darkness is only hinted at in the prequel but by the time Monsters, Inc. rolls around he's become bitter and violent.
Megamind himself is an alien who is sent to live on Earth by his parents. But he was raised in prison, unlike Metro Man, who live in a mansion with a happy family. This jealously eventually leads to him becoming a supervillain after all attempts to be good goes wrong for him.
Hal Stewart started off as a socially retarded cameraman with a creepy 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.
The Penguin is a deformed former freak show dumpster baby who becomes a charismatic gang leader bent on dominating Gotham City.
Catwoman was a nebbish secretary with no backbone and no personal life who gets murdered and resurrected as a sexy catburglar vigilante.
At the start of The Dark Knight, The Joker is being pawned off as a two-bit crook of no real concern by all sides. By the end of the movie... yeah.
Maroni: Some two-bit whackjob, wears a cheap purple suit and makeup. He's nobody. He's not our problem.
Batman: One man or the entire mob. He can wait.
In The Dark Knight Rises, there is Bane. Just another inmate of the worst prison in the world, who goes on to become an intimidating, charismatic terrorist who leads Gotham's disaffected and disgruntled in nearly destroying both Batman and Gotham City. As he puts it:
Bane: Nobody cared who I was until I put on the mask.
Benjamin Barker was a happy, ordinary London man, with just an exceptional talent at barbery. Then he was jailed on trumped up charges and sent away for ten years to Australia. He came back to find his wife had poisoned herself, and the man who did this to him had taken his daughter. Thus was born Sweeney Todd: The Demon Barber of Fleet Street.
Saw: John Kramer the Jigsaw Killer was once just a mild-mannered engineer. Then his wife miscarried their son, and shortly after that he found out he had incurable cancer. He then attempted to commit suicide, and when that failed, he decided to dedicate the rest of his life to "teaching" people to appreciate their lives. The rest, as they say, is history.
Buddy Pine from The Incredibles was just an annoying little Fanboy who refused to leave Mr. Incredible alone. Then he had one little misunderstanding with the hero. Next time we see him, he's become the supervillain Syndrome and has already murdered a large number of Supers. And that's just the tip of the iceberg.
General Bethlehem: What do you think that l did before the war?
In Blindness, the King of Ward 3 is seen, early on in the movie, as just a normal bartender who works at the luxury hotel and chats to the girl with dark glasses. Next we see him he has become a monstrous antagonist.
The Assassination of Richard Nixon is based on the real life person Samuel Byck, whose sanity is being worn out from being a loser and feeling that he is ignored by society. Eventually he tries to hijack a plane with the intention of crashing it into the White House. His mission ultimately fails, but he ends up killing a number of innocent civilians in the process.
In Limitless, a simple street loan shark is turned into a criminal mastermind when he gets hooked on the new drug that increases your mental output.
In Looper, Joseph Simmons travels back in time to prevent the rise of the Rainmaker, a mysterious individual who singlehandedly took over international crime syndicates. Eventually, he finds Cid, a boy living with his mother on a farm. Initially sweet, he will kill anyone who threatens his mother, with telekinetic powers equal in power to a small nuke. While it is implied that he became the Rainmaker in the original timeline, it is left unclear for the present one.
In the French film A Prophet, Malik starts the film as a 19-year-old illiterate loser who gets his shoes stolen from him on his first day in prison. By the end of the film, he's the boss of the prison.
In Transcendence, RIFT is mentioned as having grown from a relatively harmless group that protested the effect of cell phones on socialization. Now they're full-blown terrorists.
Played straight in the UK crime thriller Blitz. Berry starts off as a common street punk who got into it with some guys at a night club. When the police showed up, he insults the wrong one, resulting in him taking a severe beat down. Obsessed with revenge, Berry starts killing all the cops that arrested him in the past, while at the same time undermining the whole department by trying to become infamous in the media.
Scooby-Doo on Zombie Island: Simone and Lena were simple Louisiana colonists until their village was destroyed and their friends murdered by Morgan Moonscar. This drove them to become cat demons to get their revenge.
Silent Hill has Alessa Gillespie, a small girl (9 in the original, 11 in the sequel) who is burned alive by a cult, either because they believe that she's a witch or because they want to impregnate her with the cult's god. Prior to that, she may have had some psychic powers, but nothing too dramatic (an early deleted scene had her demonstrating her powers by making butterflies move in an unusual pattern, and in the film, she may have snapped a chain). Afterward, the cultists literally only survive for as long as they do because she wants them to be absolutely broken before she kills them.
Agatha, from Paranorman, had some powers during her lifetime but these didn't appear to extend beyond speaking to the dead before she was executed as a witch.
Falling Down: "D-Fens" started his day driving to his no longer existing job. By the end of the day, he's become the most violent menace in Los Angeles, attacking gangs, Nazis, and unhelpful clerks.