Fallen Heroes (or Heroes turned the Dark Side) are villains who used to be heroes but are currently evil. Either they corrupted themselves or a villain corrupted them.

These kind of former heroes had either turned evil by their own choice (e.g. Darth Vader, Lord Superman), or another villain corrupted them by force (e.g. Cat Blanc)

These kind of villains are the opposite of Redeemed villains and their Inconsistently Admirable counterparts, although they can be both at the same time, especially if they realize horrible acts they've done or in some cases in order to save a loved one (Darth Vader, Cat Blanc, Flowey, Asgore Dreemurr, Thorfinn Thorsson, Spinel, Shiroko Sunaookami (Bad Future), Saul Goodman, Goro Akechi and Kratos are examples).

Notes[]

Please note that a character must be a hero before turning evil.

• Morally neutral characters (a la "average joes") or characters initially uninvolved with any side of the conflict do not count (e.g. Willow, Thanos, Plankton, Walter White and Heinz Doofenshmirtz). They should go under From Nobody to Nightmare and/or Debased instead.
• False heroes: if they never were good, they do not count either (e.g. Senator Armstrong, Tohru Adachi, Clive Dove and SMG3). However, some false heroes in the present story can still be true heroes prior to the story (e.g. Peter Pettigrew (Books) and Goro Akechi).
• Normal public servants such as law enforcement, military, firefighters, security, healthcare professionals, etc. who became evil do not count unless they were actually heroes (e.g. Goro Akechi and Zeref Dragneel).
• Normal officials such as businesspeople, CEOs, managers, workers, doctors, scientists, politicians, school faculty, etc. who became evil do not count (e.g. Dr. Twister, Mr. Krabs and Heinz Doofenshmirtz) unless they were actually heroes (e.g. Henry J. Waternoose III and all versions of Doctor Octopus).
• Young and naïve children who became evil do not count (e.g. Beavis and Butt-Head and Eric Cartman). They should go under Kids, Teenagers and/or Juvenile Delinquents, instead, unless they are reported to have done something truly heroic (e.g. Jeff the Killer).
• Villains who simply lose all of their redeeming qualities do not count unless they were actually heroes in the past (e.g. Joy from Outside In, Jeff the Killer, and Sweet Pete). They should fall under Debased instead. Though, cases like this are extremely rare as Inconsistently Heinous villains usually have redeeming qualities, but it is not impossible, such as if said character has too much comedic moments, or has moral agency issues, etc.
• Servants/Pawns: villains who served on the hero's side but in said circumstances have never done anything heroic or good do not count either (e.g. Sideshow Bob, Agent 47, Alucard, and Henry Stickmin). They should go under Servants of Hero instead, unless they actually did genuine heroism (e.g. Darth Vader, AUTO, Kratos, Peter Pettigrew (Books), Mina Loveberry, and Goro Akechi).
• Anti-Villains/Protective: villains who do have noble and/or caring qualities but were not heroes to begin with do not count (e.g. Beerus, the Medic, Senator Armstrong, and Thanos) They should go under Anti-Villains, Vigilante, Protective, and/or Extremists instead, unless they actually were heroes who turned to the dark side and still contain their noble and/or caring traits (e.g. AUTO, Goro Akechi, Count Dooku, Zeref Dragneel, Kokushibou, Kurumi Tokisaki, Henry J. Waternoose III, and all versions of Doctor Octopus).
• Nice people: villains who were simply nice people before they became villains (e.g. Bowser (Super Mario), Gray, Gus Fring, El Macho and Herrscher of the Void) also do not count if they did nothing truly heroic before they actually became villains. They should go under Affably Evil instead, though they can be Debased, if they have lost their genuine affability.
• Famous people: villains that simply had a good reputation do not count (e.g. Balthazar Bratt and Baby-Doll). They should go under Charismatic‏ and/or Faux Affably Evil instead, unless they actually were heroes in the past. (e.g. Goro Akechi).
• Protagonists: villains who are simply the main character in the story do not count either (e.g. Beavis and Butt-Head, Peter Griffin, Stan Smith, Eric Cartman, Walter White and Mr. Krabs). They should go under Protagonists instead, the villain must be a real hero in order to qualify or formerly were actual heroes before falling into the Dark Side (e.g. Darth Vader, Eren Yeager, Mandy, Mario (SMG4), and Saul Goodman).
• Adaptational villains who are heroes or more evil villains in their original work: Villains who are simply altered adaptations of heroes or more evil villains do not count (e.g. Winnie-the-Pooh). They should go under Adaptational Inconsistently Heinous instead, unless it is established that they used to be heroes in the continuity of said adaptations (e.g. Light Yagami).

Usually they fall under debased as they were once heroes, but exceptions still lie when they redeem, so they shouldn't fall under the "debased" category as redeemed villains don't fall under them. Excellent example being Darth Vader.