Sudden Sequel Heel Syndrome

So your story was a smash success, and you've gotten enough interest or capital to do a sequel. However, you fully explored all of the characters in the first story and have nowhere left to go with them, and you think it would stale your writing ability to retread the same characters and events from the first story. So you need a way to not only stretch your storytelling chops, but also hand out enough Shocking Swerves to keep the audience surprised. How do you do this?

Why, by turning one of the main good guys into a bad guy!

When done correctly, Face Heel Turns can be shocking, compelling and tragic. Few things tug at the heartstrings like when a cutie is broken or when The Paragon crosses the Despair Event Horizon and goes bonkers over the unfairness of it all. When done poorly, the turn comes out of absolutely nowhere or betrays the expectations that had been set by the character's portrayal up til then. Audiences tend to become attached to their heroes and some become moreso "icons" than characters, leading to the audience feeling betrayed by not only that character, but the writers themselves.

If this is a prequel, and an enemy from the original is part of the group, then this is Doomed By Canon to happen.

A subtrope of Face Heel Turn. Separate from (but not mutually exclusive to) Sudden Sequel Death Syndrome.

Super Trope of Rogue Protagonist, Fallen Hero, and The Paragon Always Rebels. See also Ron the Death Eater for fanfiction, and Adaptational Villainy.

WARNING: May contain spoilers.

Anime/Manga

 * Joker, the head of the British Library special forces in Read or Die. Between the OVA, where he's on the side of the good guys, and R.O.D The TV, the death of Gentleman (the Man Behind the Man of hundreds of years of British history who envisioned a sort of Utopia under the British Empire) shifts Joker into Well Intentioned Extremist territory, working toward what he sees as Gentleman's dream by means ranging from ethically dubious to outright evil.
 * Yu-Gi-Oh Season 4 turned Mai Valentine, the gang's Cool Big Sis, into an angst-ridden member of a villainous biker gang. One reason among many that the season falls squarely into Fanon Discontinuity for many fans.
 * It turns out she was tricked into Heel Face Brainwashing by the arc's Big Bad, Dartz.

Comics

 * Comics in general loves a Face Heel Turn, but usually don't qualify for the trope unless the character gets Put On a Bus for a while and returns evil. A few notable examples include:
 * Jean Loring in Identity Crisis, who went completely off her rocker to try and win back the affection of her ex-husband, Ray Palmer.
 * The survivors of Crisis On Infinite Earths (Superboy-Prime, Superman (Kal-L), and Alexander Luthor) all became contemptuous of the Modern Age DC Universe and launched a scheme that would forcefully return things to the Silver Age status quo, whether the universe was ready or not.
 * Linda Danvers retired from superheroics after Many Happy Returns, but returned in Shadowpact, transformed into a vengeful "Fallen Angel".
 * Cassandra Cain during DC's One Year Later storyarc.

Film

 * Jim Phelps in the Mission Impossible film franchise.
 * In the first Robo Cop, the The CEO of Omni Consumer Products is merely an amoral old man who really doesn't do anything outright villainous, but shows little empathy for others. In the sequel, he's a flat-out Corrupt Corporate Executive.
 * Ug in Critters 4.
 * Tron Legacy does this to
 * Star Trek VI the Undiscovered Country turns Admiral Cartwight from Star Trek IV the Voyage Home into a co-conspirator with the bad guys.
 * in the Aladdin films.

Literature

 * In the first story of G.K. Chesterton's Father Brown series, detective Aristide Valentin of the Paris police force is a likable Inspector Lestrade to the title character, totally fooled by Father Brown's Obfuscating Stupidity but competent enough to follow the trail of clues the little priest leaves for him. In the next story, however, he becomes a Straw Atheist who murders a man to prevent him from leaving money to the church in his will. (And, for extra irony points, the villain from the first story is given a Heel Faith Turn to become The Watson...)

Live-Action TV

 * Sikozu in Farscape: The Peacekeeper Wars.

Videogames

 * The Masamune sword in Chrono Cross. In the Chrono Trigger, the Masamune is a coveted magical weapon which is said to be the only the only weapon capable of hurting the wicked sorceror, Magus. It's outright said many times that only the Legendary Hero can wield the sword and at one point, said Hero even goes through a Secret Test of Character which allows the sword to recognize him as its true owner and unlock its full potential. Then, in the sequel, the Masamune is explicitly stated to be an "evil" sword and anyone who wields it will immediately go insane. However, this is Hand Waved in that the Masamune is sentient and inhabited by two mischievous young spirits. When their big sister shows up and smacks some sense into them, the Masamune becomes good again.
 * Sigint, Para-Medic and Major Zero in Metal Gear Solid. It can be argued that their creation in the first place was to explain the Start of Darkness which lead to the creation of "The Patriots", but few can argue that it's a shock to learn that the diabolical "Dr. Clark" we heard about in the first game was the sweet and flighty Para-Medic.
 * Metal Gear Solid 2 Sons of Liberty cheerfully subverts this - the villain claims to be Solid Snake, but he clearly isn't.
 * Diablo II: the protagonist of the original is now the Big Bad.
 * Justified in that at the end of the first game the protagonist
 * Lloyd Irving in Tales of Symphonia Dawn of the New World. Well, sort of.
 * In the Wing Commander video game series, the game Wing Commander III: Heart of the Tiger had.
 * Likewise, the Big Bad of Wing Commander IV: The Price of Freedom
 * This happens to heroine turned Action Mom, Sophitia Alexandra, in Soul Calibur IV; where she's made to defend Soul Edge in order to save her daughter's life.
 * It also happens to the eponymous Soul Calibur itself in the fourth game; when the sword reaches full power and gains sentience, it's revealed that the Calibur, long thought to be the "good" sword throughout the series, is technically just as "evil" as the Soul Edge. The difference is that the Soul Edge likes to Rape Pillage and Burn in order to spread chaos and misery while the Soul Calibur wants tranquility, order and peace...by freezing the entire world solid!
 * Vladimir Lem in Max Payne 2 The Fall of Max Payne. Although it should be noted it's less that he went evil (he was already a Mad Bomber Mob boss), and more that Max Payne ends up on the wrong side of Vladimir's machinations this time.
 * Contra: Shattered Soldier reveals that Blood Falcon is actually Lance, who was player 2 of the original game. He went insane trying to reveal the Government Conspiracy he and Bill were duped into helping cover up.
 * In a conscious effort to avert Bag of Spilling and Restart At Level One, the developers of Prototype designed a new protagonist for the sequel and made Alex Mercer, former protagonist (albeit not hero), into the Big Bad. Unfortunately, they did this by putting all his hard-won Character Development into hard reverse without any ingame explanation whatsoever, and the out-of-game reasoning seeming forced and steeped in Fridge Logic considering the Character Development. Indeed, so few plot threads or character traits are carried over from the previous game you'd be forgiven for assuming the new game is set in an Alternate Universe or something.
 * World of Warcraft has gone hard on various characters from the Warcraft Expanded Universe in order to turn them into bosses which may be killed for loot. Perhaps the most jarring case of this would be transforming the Anti Hero Illidan and his allies Kael'thas Sunstrider and Lady Vashj into villains with little redeeming quality.

Western Animation

 * In Beast Machines,