At the end of Disney's animated The Great Mouse Detective (see the image on the right), Ratigan goes from being a smarmy, smug intellectual to a feral, crazed rat, savagely attacking Basil. Although he does have quick seconds of losing his cool throughout the film, he is just as quickly able to recover and be calm and collected. But when Basil both humiliates Ratigan in front of dozens of aristocrats and saves and reunites Olivia and her father (whom Ratigan had been keeping separate for most of the movie), Ratigan breaks down.
This is a particularly interesting example, as rather than suffering from Villain Decay because of it, this scene is made one of the more terrifying in the film, and even becomes the closest this villain gets to actually killing the hero.
In 101 Dalmatians, Cruella De Vil's sanity slowly whittles away over the course of the film, as her obsession with making a fur coat out of the Dalmatian puppies evidently goes to her head. It begins after the Radcliffes refuse to sell her the puppies and she makes it clear to them that she is Not Good with Rejection. It continues to erode when she becomes impatient with her Mooks, Horace and Jasper, to get the job of skinning the dogs done, and, soon after, when they discover the dogs missing and have to hunt for them. And she will not take no for an answer. Eventually, what little sanity remaining in Cruella is completely spent during the climatic Chase Scene. The fact that she has completely lost it becomes especially acute after she ragingly drives her car out of a ditch and through some thorny bushes, trashing the car pretty good in the process. Clearly, by this time, it is either the dogs or her — and she doesn't care who gets in her way. That's how much she wants that fur coat. And when her mooks' rickety old truck accidentally t-bones her already-trashed-car, destroying both cars completely and leaving Cruella and her henchmen adrift amid the wreckage, Cruella ravingly bawls them out and breaks down sobbing, to which Jasper has long since had enough and tells her, "Ahhh, shaddup!"
The Little Mermaid: Ursula towards the end after Ariel destroys her eel pets, effectively triggering her One-Winged Angel transformation, which involves her growing to GIGANTIC size and speaking in adeepvoice. Also, when she transforms into Vanessa, it is implied that she lost quite a bit of sanity (to the point of becoming a borderline Ax-Crazy) when turning into her, as she talks to her mirror in a manner similar to a schizophrenic, emits a psychotic grin when throwing a pin at a mirror's head with enough velocity to knock the mirror back, and most certainly tries to kill a person had that been a human being, and her cackling.
Sykes in Oliver & Company. While he's mainly calm throughout the movie, in the climactic car chase he pulls down the gear stick so hard it breaks off, and punches his hand through his car's window to get at the heroine.
Jafar has this after learning that Aladdin took back the lamp at the cave of wonders. And then there's his Laughing Mad moment after the Prince Ali Reprise, his spouting off his Hurricane of Puns during the climactic battle, and his Big "NO!" when he realizes that he's a slave to the lamp after becoming an all-powerful genie. Subverted earlier on when Jafar starts laughing maniacally after being exposed for the Evil Chancellor he is. Iago thinks he's cracked, but in reality, he realized that Prince Ali is actually Aladdin.
In Wreck-It Ralph when King Candy is revealed to be the missing World Hopping character Turbo, he flips out as he attempts to kill Vanellope during their climatic race.
In Atlantis The Lost Empire, as Rourke attempts to escape with the MacGuffin in the midst of an attack by Atlantean fliers, he slowly loses the calm demeanor he had sported throughout the rest of the movie, sporting one hell of a Slasher Smile and attacking Milo with a fire axe.
Rourke: Well, I have to hand it to you, you're a bigger pain in the neck than I ever would have thought possible. I consider myself an even-tempered man; it takes a lot to get under my skin, but congratulations! You just won the solid gold kewpie doll!
AUTO in Wall E sums up the trope with one sequence. The captain has raised the Holo-Imager on the lido deck, so AUTO knocks him aside and hits the button to retract it. WALL•E blocks it from retracting with his body. AUTO hits the button again, and when it still hasn't gone down, he hits the button so hard and rapidly it cracks, and then tasers it. When the Captain advances on AUTO, his taser is turned on him.
Hopper in A Bugs Life has two: first, when the fake bird is exposed and the ants then stand up to the grasshoppers; second and more acutely, when his taking Flik hostage is thwarted by Atta and Hopper gives chase, viciously pursuing Flik to an actual bird's nest, where Hopper doesn't think that the birds are real until it's too late.
Warning: System Overload Hein: I know. Warning: System Overload Hein: I know! Warning: System Overload Hein:I! KNOW! But it must be done.
After the predictable happens, Hein just sits at the firing control while the whole damn thing explodes around him, fruitlessly attempting to get the doomed superweapon to fire one last shot. Then the primary bridge crashes into the room he's in.
In Kung Fu Panda, Tai Lung's composure really starts to fall apart once he sees the Dragon Scroll he so coveted. Instead of fighting intelligently and using environment to his advantage, like he did in every fight before, he single-mindedly pursues the scroll, losing more and more of his cool, as Po starts to give him problems. After he obtains the Dragon Scroll and finds it to be blank, he suffers from this full out, and it just gets more intense after he discovers his pressure point technique to be ineffectual against Po, causing him to throw all semblance of strategy and martial arts mastery out the window, and after getting beaten and barely able to get up and stumble around, he just keeps rambling and trying to fight back.
And in the sequel, Kung Fu Panda 2, Po appearing as Shen is about to sail to triumph seems to result in the peacock finally losing it; while he remains sane, he kicks the Attack! Attack! Attack! strategy he'd employed with his cannons up to 11, even willing to fire on his own fleet to clear out all obstacles and casually knifing The Dragon when he refuses. This results in Shen refusing to cease using his cannons even when Po has perfected the catch and return technique, resulting in a pretty epic Oh Crap! when Po's final returned shot makes a yin-yang symbol before striking his flagship. When Po confronts him on his ship after crippling it, he finds Shen completely stunned by both the fact everything he created has been destroyed and the fact that Po managed to overcome his traumatic past and find inner peace. When Po explains it to him, he snaps and tries to kill Po. Unlike Tai Lung, however, Shen doesn't lose his head, managing Tranquil Fury despite his breakdown, resulting in a much more even fight.
This is most likely due to the Soothsayer's prediction starting to come to pass and by this point Shen is getting desperate to change it. His Mind Screw didn't work and force is his only option. You can sense it in Shen's voice when he ask how Po overcame his trauma. Even if its calm, his ambition has been left in ruins and he's got nothing to lose. Leading to the final assault and ultimately death at his own hands.
Darla Dimple of Cats Dont Dance gets a big one at the end. Darla's attempts to sabotage Danny's last-ditch effort to show that animals can be stars actually helps them. At the end, frazzled, exhausted and more than a little mad, Darla crawls up to Danny and, in front of the audience, makes the fatal mistake of outing herself as the one who sabotaged his earlier attempt.
Darla: I... should've drowned you all when I flooded the stage!
Lord Barkis Bittern from Corpse Bride after finding out that Victoria is poor after he married her and especially at the end where he has a major Freak Out moment after he dies from accidentally drinking poison and the dead attack him.
One of the few cases where this actually happens in the first movie and leads into the second: villainous cockatoo Nigel loses his feathers in Rio and ends up suffering all sorts of humiliation as well as not being able to fly again. The minute he sees Blu and Jewel again in Rio 2, he snaps (complete with flashback) and spends the entire movie going to all sorts of lengths to hunt them down.
Chicken Run: As Mrs. Tweedy tries to stop the chickens from escaping, wielding an axe, Rocky knocks her out and the chickens all manage to escape. But when Mrs. Tweedy comes to, she lets loose with primal rage and goes Ax-Crazy in her determination to stop the chickens in general and execute Ginger in particular with the axe. And after they escape for good, when Mr. Tweedy sheepishly tells his wife, "I told you they was organized," Mrs. Tweedy simply roars at her husband through gnashed teeth, as if to say, "I hate it when you're right!"
Coraline. The Other Mother is normally a creepy Stepford Smiler. She loses her fragile composure after Coraline refuses to apologize to her and after Coraline throws the cat at her face and it claws off her button eyes. Also, when Coraline is escaping to the real world from the other world the Other Mother loses it and screams "DON'T LEAVE ME! DON'T LEAVE ME! I'LL DIE, I'LL DIE WITHOUT YOU!!!!!!" Also, after they throw the hand and the key inside the well, if you listen closely, you can hear the Other Mother whispering 'No'.
Synoamess Botch from Twice Upon a Time goes absolutely ballistic when his evil plot is foiled by Ralph. He then becomes terrified to the point of begging for his mother by the threat of a leftover nightmare bomb going off in his face. Said bomb is really Ralph's buddy Mumford in disguise.
Dark Fury. After Riddick has killed most of Antonia's minions and foiled her plan to have him turned into part of her art collection, she attacks him in a shrieking frenzy and noticeably dishevelled before Jack kills her.
The Prince of Egypt: In a very tragic example, Rameses snaps after the death of his son, riding after Moses and the Hebrews and, when they try to escape him through the Red Sea, he shouts to his men "Kill them! KILL THEM ALL!" The last of him seen in the film is him screaming in rage and agony, cursing Moses.
In My Little Pony: Equestria Girls, Sunset Shimmer is looking very uncertain, like she might even be open to a truce, after Twilight Sparkle calls her bluff — at least until Twilight's friends start praising her as the sort of leader Sunset wanted to be regarded as, causing her to snap and forcefully wrench the crown from Twilight and her friends. It's also implied her Villainous Breakdown was part of the reason the crown transformed her into a demon upon her putting it on her head, as her own rage and resentment at Twilight finally came to the front full-force and caused the Element of Magic to reflect what it saw within her heart. If nothing else, it was certainly the point she took several levels in insanity and evil, going from someone who refused to harm Spike to willingly attempting to killTwilight with a fireball.
Ice Age 4: Continental Drift After Manny and the gang steal his ship to get back home, Captain Gutt is so furious, it's quite obvious he's lost a huge amount of his sanity. He later beats Manny and the gang to the continent and takes Ellie & Peaches hostage, stating that Manny took everything he had from him, and the evil ape intended to do the same.
Later during the battle when Precious vanquishes his crew, only Gutt dodges them, now not caring about his crew. Then he attempts to kill Ellie but not before Peaches kicks him to the wall. Finally losing what's left of his sanity, Gutt then blocks Manny's path to return to his family, just as a landmass sends both of them hundreds of feet high.
Gutt: Told ya, Tubby. You shouldn't have messed with the Master of the Seas!