Revealing the Monster Within

One of the most common moments in storytelling, there comes a time when the villain reveals the monster within which is an opposite to revealing the angel within. In other words, this means he or she reveals his or her own true colors, usually in the third act, the final climax, or in any other scene. The evildoer can reveal the monster within in many ways:
 * Pulling off an unexpected wicked act.
 * Betraying the hero.
 * Taking part in the final battle.
 * Going through a villainous breakdown.
 * Having true sinister agenda unexpectedly exposed by the hero/heroine, particulary if the said hero/heroine was wary with them.
 * NOTE: This should be not confused with the One-Winged Angel scenario where a villain literally turned into monster, but this sometimes overlapped had as the said villain revealed their true color, unexpected circumstances (be it either against the said villain's wishes or villains' own capability to turned into monstrous forms) led them to taking more hideous form.

Examples

 * After promising Pinocchio he will make him a star, Stromboli reveals his true colors when he locks the puppet in a cage and plots to make him perform in his show.
 * Gwen Grayson accepts her award in Sky High, but then reveals herself as the wicked Royal Pain in front of the audience.
 * After Danny disrupts her stage performance, Darla Dimple flies into a rage, revealing herself as the vile little girl she truly is.
 * Prince Hans reveals his true colors when he betrays Anna.
 * Dawn Bellwether reveals the monster within when Judy Hopps discovers she is the mastermind behind the savage attacks.
 * Sloan and Bree Blackburn reveal themselves to Eliza Thornberry to be not kindhearted zoologists as they initially appeared, but actually evil, greedy and sadistic poachers who kidnapped Tally, the cheetah cub Eliza tried to save, and also plan to kill a herd of elephants for their ivory with an electric fence.
 * Thunderclap reveals the monster within after Arlo seeing him, when he devours a critter.
 * Mrs. Quivers unveils the monster within when she reveals herself as Madison's kidnapper and her plot to release her imprisoned husband and take over the world.
 * Koba shoots his former friend, Caesar, so that he could become the leader of the apes and start the war between humans and apes.
 * Stinky Pete reveals the monster within after he traps Woody, Bullseye, and Jessie in Al's apartment and also reveals that he framed Jessie for turning on the TV last night.
 * Archibald Snatcher reveals the monster within when he plots to wipe out the boxtrolls in Cheesebridge.
 * Henry J. Waternoose III reveals his true colors when he exiles Mike and Sulley to the human world and kidnaps Boo in a bid to revolutionize his company.
 * Noah Cross reveals the monster within when he reveals his plot to rule Los Angeles and kidnaps Evelyn Mulwray's daughter.
 * Robert Callaghan reveals the monster within when Hiro and the others discover that Callaghan is Yokai.
 * Harvey Dent reveals the monster within when an unfortunate accident disfigured the left side of his body, becoming known as Two-Face.
 * Sailor John reveals the monster within when he reveals to Thomas he plans to keep the treasure for himself
 * Lysandre reveals the monster within when he and his organization began their assault on Lumiose City with Z-2.
 * Creek reveals the monster within by selling Poppy, Branch and other trolls to Chef and other Bergens.
 * Josh reveals the monster within after Tommy want to play "acrobat" instead of "digging around the jungle gym" in Rugrats episode "New Kid in Town".
 * Ray Thompson reveals the monster within when he betrays the Justice League and the JGA in the Justice League episode "Legends".
 * Miranda Frost reveals the monster within as she reveals that she was the one who betrayed James Bond.
 * Commander Rourke reveals to Milo Thatch his true intention to steal the Heart of Atlantis, not even caring that the Atlantean people will die without it.
 * Ben Ravencroft reveals his true nature by admitting to Mystery Inc. that he used them to find his ancestress Sarah Ravencroft's spellbook.
 * Noo Noo (Slendytubbies) reveals to White Tubbie that it caused the infection.
 * Ernesto de la Cruz reveals the monster within when it is revealed that he poisoned Hector and stole his songs.
 * Tobias Beckett reveals the monster within as he betrays his former friend and pupil Han Solo.
 * Eli Mills reveals the monster within when Owen and Claire discover that he plans to sell the dinosaurs for profit.
 * Dave York reveals to McCall what he and the CIA got involve his dirty buissness and killed his neighbor to prevent her from finding out the truth.
 * Clancy Gay reveals the monster within when he attempts to rape Ruby.
 * Cecil Fredericks, Reginald and Gus reveal their true nature when Larry finds out that they had used him and set him up before stealing the Tablet of Akhmenrah.
 * DJ reveals his true nature when he betrays Finn and Rose.
 * Clayton reveals his true nature to Tarzan, by using him to reveal the gorillas' location to Clayton, so he can capture them all and sell them to zoos to make a huge profit.
 * Zobek reveals his true nature to Gabriel Belmont by putting on the Devil Mask, and told him that he’s been manipulating him from the very beginning.
 * Evelyn Deavor reveals her true super hero-despising self after being unmasked as Screenslaver.
 * Palpatine reveals his true colors to Anakin Skywalker during the climax of Revenge of the Sith.
 * Elsa Schneider betrays Indiana Jones that she was a Nazi who wants the journal that will leads them to the Holy Grail.