Revealing the Monster Within

One of the most common moments in storytelling, there comes a time when the villain reveals the monster within. In other words, this means he or she reveals his or her true colors, usually in the third act, the final climax, or in any other scene. The antagonist can reveal the monster within in many ways:
 * Pulling off an unexpected evil act
 * Betraying the hero
 * Taking part in the final battle
 * Going through a villainous breakdown

Noteworthy Examples

 * After promising Pinocchio he'll 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 the monster within after Eliza Thornberry rescues Tally.

Quotes
"Hello, Eliza. (Eliza spins around. Sloan is standing behind her in the doorway, smiling wickedly. Tally wriggles in her grasp.) (Eliza: Sloan? This cheetah cub, where--where did you get him?) I picked him up on the savannah. (Eliza: You bought him?) (Sloan laughs.) No. I picked him up...right off the ground. (Bree walks to Sloan's side with a nasty sneer.) (Bree: I told Sloan he shouldn't CUT that rope ladder!) (Eliza, with a gasp: It was YOU!) Yes. (Sloan pulls his knife out of his boot.) Thank you so much for returning my knife to me. (Bree, with mock-sympathy: Oh, kitten, you didn't drink all your milk.) (As Bree speaks, she roughly yanks Tally out of Eliza's arms. Tally struggles to get free.) (Bree: How are you ever gonna grow big enough to make me a new coat?) Oh, I'm afraid you're gonna have to miss the solar eclipse. (laughs) You can catch it on the Poaching Channel.) (Eliza: You build a fence!) Of course. Now here's a question for your famous father: How many volts of electricity does it take to kill a thousand elephants? Hm?"

- Sloan Blackburn revealing the monster within. "(Judy Hopps and Nick Wilde enter the Natural History Museum and have reached a shortcut to the ZPD.) (Judy: There it is!) (Just then, a voice rings out.) Judy! (Our heroes spin around. It's Bellwether, flanked by two big rams, and she seems excited.) Judy! (Judy: Mayor Bellwether!) (Judy and Nick run over to her.) (Judy: We found out what is happening. Someone's darting predators with a serum. THAT'S what's making them go savage.) I'm so proud of you, Judy. You did just a SUPER job. (Judy: Thank you, ma'am.) (She prepares to hand the gun case to Bellwether, but grows surprised.) (Judy: How did you know where to find us?) (Bellwether reaches for the gun case.) I'll go ahead and take that case now. (Judy, pulling the case away: You know what? I think Nick and I will just take this to the ZPD.) (The two of them turn to the door, but a hulking mountain of a ram stands over them, cracking his huge neck. That's when the pieces fall into place.) (Judy/Nick: Run.) (The two of them make a run for it.) Get them."

- Dawn Bellwether revealing the monster within.