Viserys Targaryen, after it becomes clear that he will get a rather different "crown": "You cannot touch me, I am the dragon!"
Ser Gregor Clegane, as he fights Oberyn Martell, gets progressively more enraged. A combination of his exceptionally dim wits and the realization that he is being publicly accused of his crimes in front of the entire court leads to him screaming "SHUT UP!" at Oberyn again and again. Even killing Oberyn doesn't help, because the spear that Oberyn wounded him with during the fight is coated in poison, leaving Clegane to die, slowly and painfully, over the course of days.
Ser Amory Lorch, after the Brave Companions and the Northmen take his castle, reveals himself to be a Dirty Coward and weeps and cries before he is eaten by a bear.
Ser Jaime Lannister remains his usual composed, snarky, debonair, and uncaring self despite being defeated in battle and imprisoned. But when he loses his sword hand, he goes completely to pieces, and loses the will to live. He gets better thankfully fast, with a little help, which leads directly to a Heel-Face Turn.
Lady Lysa Arryn, when she sees her childhood crush forcibly kissing Sansa Stark who is her niece, goes completely insane, trying to throw poor Sansa (who did not instigate the kiss) out of the Moon Door. Her breakdown is cut short when she is thrown out said Door herself.
Craster, a particularly nasty wilding who "marries" his daughters and sacrifices his sons to the resident Eldritch Abominations, takes the suggestion that he is of dubious parentage extremely poorly: "WHO CALLS ME BASTARD?"
Septon Utt, a Pedophile Priest and child murderer frantically pleads with the Brotherhood Without Banners for his life. It doesn't help.
Merrett Frey, a member of the universally despised House Frey, is told to deliver a bag of gold to ensure the release of one of his slimy relatives. He brings the gold, his relative is hanged anyway, and he is next. He takes it badly.
Janos Slynt, the captain of the King's Landing city watch who betrayed Ned Stark when he was about to end the whole Lannister conspiracy in the first book, gets two. First, when Tyrion promptly strips him of all the rewards his betrayal got him and sends him to the Night's Watch, and then when he refuses to obey a minor order from the Night's Watch's newly elected Commander Jon Snow. Jon declares that he'll be executed, but Slynt still doesn't take him seriously. Then he's dragged to the block and realizes Jon actually intends to go through with it, and starts pathetically begging for his life. It doesn't help. The latter is one of the series's most satisfying moments.
Only seen in backstory, but Aerys Targaryen, the Mad King, was all set to burn all of King's Landing to the ground with strategic stores of wildfire, spending his last hours just saying "Burn them all" over and over again. Then Jaime happened.
Although he hasn't been the most composed guy over the course of the book, Grevane in seventh book in The Dresden Files Dead Beat by Jim Butcher has a Villainous Breakdown that saves Harry's (or maybe Ramirez') life - right at the instant of his death. The entire time he's been trying to fight Harry he's been continuing a major necromantic working, and when Ramirez strikes the killing blow, his mind snaps and refuses to accept that he's dead. As a result, he keeps trying to finish the spell, instead of casting a Death Curse that would take Harry with him.
Arianna Ortega experiences this in book 12. Throughout the book she's been a Smug Snake, and the fact that she plans to murder a little girl purely for selfish reasons only adds to this. Eventually she faces the father of her would be victim in single combat and ultimately looses badly, getting impaled by numerous ice spears (including one as thick as a man's fist.) As Harry stands over her she simply looses her cool and first says "no no no" over and over again before telling Dresden that he is cattle. He replies "Moo," and blows her head off.
Her dad the Red King also suffers an implosive one. After getting heavily injured and overpowered by Harry, he first summons all his soldiers despite earlier showing fear at the prospect and he is reduced from a Smug Snake Chessmaster into a screaming lunatic who inadvertently cuts one of his deputies in half.
Gonko, the normally cool-headed leader of the Clown Division has one halfway through the book, when he discovers that someone has stolen his trousers — which are, in fact, enchanted to produce any number of useful items via the pockets. However, since most of the clowns seem to be insane to a certain degree, this is less of a mental breakdown and more of an explosive temper-tantrum. Nonetheless, whilst obliterating all furniture within reach, he screams this:
Gonko:If I find the motherfucker who took my pants— I DON'T CARE WHO YOU ARE: CLOWN, ACROBAT, BELOVED FRIEND OR RELATIVE, AN INANIMATE OBJECT... AN ASTRAL BODY... ME MYSELF... A ROCK OR A BOWL OF PICKLES... SOMETHING UTTERLY IMPOSSIBLE TO KILL, LISTEN UP: I'LL FUCKING KILL YOU! I'LL FIND A WAY, IF IT TAKES ME A HUNDRED YEARS... I'LL FIND... A... WAAAAAYY!
Mugabo, the Circus Magician, apparently has one every once in a while: being a gifted sorcerer and pyromancer he feels degraded at having to perform the "bunny trick," and breaks into fiery temper-tantrums when pushed into it. However, when the Freedom Movement sabotages his tent and scrawls graffiti all over his equipment near the end of the book, he enters a fully-fledged pyrokinetic psychotic episode, resulting in the death of several carnies and the utter astonishment of the visiting tricks.
One of the creepier moments in Neil Gaiman's Neverwhere comes when the Angel Islington, who has been nothing but calm, reserved, and beatific, wheels on Richard and thunders, "THEY DESERVED IT." when asked about what happened to Atlantis. The book compares it to lifting the lid on a container and seeing something writhing underneath. Richard notes that "in a time of scary things, it was the scariest thing [he] had ever seen."
Emperor Jigang goes through one in Pillars of Creation when, upon arriving at the Confessor's Palace in the Midlands capital, he finds the head of his friend and mentor, stuck on a pike in front of the palace, perfectly preserved by a magic spell until he approaches it, whereupon it decomposes in seconds.
In Murder on the Ballarat Train, the killer finally let go with one of these complete with a Nietzsche Wannabe speech.
In Queen of the Flowers, the two thugs whom Phryne's friends had been tracking down for information were soon being hunted by a ruthless mob boss, thanks to some remarks she'd made about them. When they later turned up, one got peacefully drunk, but the other went into a screaming tantrum while waving a loaded shotgun.
Gabool the Wild in Mariel of Redwall does it most obviously and impressively. He goes from being evil but reasonably lucid to a gibbering insomniac who can't tell his followers from his sworn enemies and starts to believe that a plundered church bell understands what he's saying and rings itself to mock him. Of course, it might actually have been ringing itself to mock him. It's not clear.
Slagar the Cruel in Mattimeo is already crazy at the start, blaming Matthias and the Redwallers for the horrible scarring on his face. By the end, he's raving, frantically reassuring himself that however events turn out, he will "win" somehow. He even plans to steal Matthias's sword, now convinced that it is magic and grants victory to whoever wields it.
Tsarmina in Mossflower also does this. Granted she's being driven insane by a constant dripping noise and the fact that everything she tries to destroy the resistance fails.
Randall Flagg from Stephen King's The Stand goes through a slow but steady Villainous Breakdown about midway through the book. A series of mishaps, such as his agents' bomb failing to kill all the leaders of the Free Zone, his destined wife goading him into killing her, his attempts to deal with the Free Zone's spies all failing in one way or another, and his idiot savant weapons expert going nuts and blowing up all his jets and pilots all take their toll on his Dark Messiah persona. Near the end, when the same idiot savant brings back a nuclear warhead that's leaking radiation, interrupting Flagg's staged execution of two Free Zone members that was meant to boost the morale of his followers, he's so unhinged that he's reduced to pathetically whining to his right hand man to get rid of the warhead.
So Tropetastic it's featured at least twice thrice several times in the Discworld:
Wyrd Sisters had Duke Felmet, who becomes more and more unhinged as the story progresses from the guilt of killing his brotherfor the throne of Lancre. And from the fact that everyone seems to know, but not care. The citizens of Lancre are more the sort to think that king being killed in usurpation really doescount as "Natural Causes" than the sort to rise up in arms over a beloved ruler's death only to be tragically put down. They don't even seem to care that Felmet and his wife are ruling the land with an iron fist: "You couldn't oppress a people like that any more than you could oppress a mattress."
Reacher Gilt in Going Postal goes through a subdued form of this trope, going from the perfect con man to someone realizing he's been duped by the ultimate con.
And then there's Cosmo Lavish in Making Money. He finally snaps for good right at the end, but it has already become evident as the story progressed that he's been going odd in the head from the start, even while his plans still seemed to be working. He's teetering on the brink and only needs a minor push to go over the edge. The whole thing is symbolised by the finger upon which he wears a ring that's far too small, which starts to sound more and more worrying, until near the end when his glove comes off — what's happened to the finger is revealed to be much the same as what's happened inside his head.
Count Magpyr in Carpe Jugulum is a polite, urbane sort who is almost able to convince you that he really is working for the common good in having people queue up to have their blood drunk. After he's been Weatherwaxed, and the villagers turn on him, he loses his calm façade and reverts to the bloodthirsty monster that he really is, until he reaches the point of threatening babies and killing dogs.
Salzella from Maskerade goes on an increasingly unhinged Motive Rant upon being revealed as the killer, adding more and more exclamation points to his sentences until he reaches five, earlier noted as a sure sign of madness. His final words are about how the thing he hates most about opera is how people take forever to die, which, of course, is just what he's been doing - taking forever (and ranting all the way).
Teatime in Hogfather is quite put off his game when Susan lays out what a sick bastard he is, until he's screaming at his own men, not realizing he's hit the Berserk Button of one by insulting his mother.
Happened to the villain of Illusion by Paula Volsky. The book is a fantasy loose retelling of the French Revolution and the Terror. As the Robespierre Expy loses his absolute control, he loses his sanity.
Prince Xizor in the Star Wars Expanded Universe novel Shadows of the Empire. At numerous earlier points in the novel, Xizor is shown to consider himself superior to "mere humans" because his cold-blooded nature leaves him incapable of going into an uncontrollable rage. Turns out it's not that he can't, just that it takes a lot longer. As his palace collapses underneath him and Darth Vader lets him know that he's quite aware of Xizor's attempt to kill Vader's son, he starts to get a little... antsy.
Tarkin has a very brief one at the end of Death Star, when he is simply refusing to accept that his prized station has a weakness even when Luke has fired the kriffing torpedoes through it. Essentially, it boiled down to "Unthinkable. Unthinkable - " Kaboom.
A very, very minor one in The Thrawn Trilogy. Grand Admiral Thrawn was able to anticipate and counter almost anything the Rebellion and anyone else did. Almost. His reaction when those things he hadn't anticipated all came together to thwart him was to get maybe a little flustered, a little distracted, enough so that he didn't see the Bodyguard Betrayal until it was too late. Though, to his credit, after being stabbed he smiled and said "But... it was so artistically done."
Thrawn's fellow Big Bad, Joruus C'baoth, has a more traditional one after Mara kills the Luke clone. Somewhat unusually, the normally unstable C'baoth actually becomes much calmer and more controlled as he's in the process of finally losing it. This just makes it creepier.
The real bad guys, at least for the characters in the main plotline, of the Hand of Thrawn duology are the Big Bad Triumvirate of Moff Disra, Major Grodin Tierce, and the Con Man Flim. They work to make it look as if Thrawn has come Back from the Dead. When Pellaeon exposes their whole plot at a crucial moment, Tierce has an incredibly sudden, violent breakdown/Motive Rantwhere it's revealed that not only is he secretly a clone, but a clone with a bit of Thrawn engineered into his brain. If you read carefully there are hints of this, like the way he very slowly changes over the course of the novel and the way he comes up with tactics becomes creepy.
Happens to Ysanne Isard in slow motion during The Bacta War. She's out of her seat of power, her Xanatos Gambit hasn't gone quite to plan, then someone breaks out of her secret prison, "defiling" it, and she starts letting things get personal. As the Rogues continue to accumulate victories, parts of her empire start splintering off, and one of her Star Destroyer captains sends her a "The Reason You Suck" Speech explaining why he's defecting to the New Republic. The Dragon notices her changing, and she notices it too. She claims to have recovered by Isard's Revenge, but even then she's still quietly obsessing over her defeat, letting her enemies predict her ultimate goal and stop her for good.
General Melvar: Will you be wanting your office restored, or do you wish to redecorate?
Karos Invictad in Reapers Gale, seventh installation of the Malazan Book of the Fallen as Tehol Beddict continually outwits him, really losing it when Tehol demonstrates his superior intellect by solving a puzzle he deemed impossible (and that Tehol created, incidentally).
In The Saga of Seven Suns, Basil Wenceslas is a classic of this trope. When things stop going according to plan, they Ski Down The Slippery Slope. I call it thus because they stop just short of jumping off entirely, but still make an impressively rapid descent.
In The Riddles of Epsilon,The Lady Yolande goes through one of these, betraying the proper choice as it screams at the protagonist to quit hesitating and choose one faction or the other, and threatening to kill the protagonist if she doesn't give it the tooth.
Harry Potter always has Voldemort close to the edge but somehow maintaining his cool... until Deathly Hallows at least, where Voldemort has so completely ripped his soul apart from his repeated killing and use of Horcruxes that he completely loses it on several occasions, at one point killing everyone in the room because a helpless goblin delivered some bad news. Even when he BSOD's during the final fight against Harry he transfers the shock of that into rage and denial.
Also, Bellatrix, too. Though she was insane to begin with, throughout the series, she just loses it, especially in the final battle. Especially taunting Mrs. Weasley, who was living on grief and adrenaline, producing the ever-famous "NOT MY DAUGHTER, BITCH!!!" reaction.
In Harry Potter and the Order of the Phoenix, Dolroes Umbridge has one at the end. First after she used up Snape's fake veritaserum, and he tells her making more will take a month Again, when she is is tricked into going into the forest and is attacked by a herd of angry centaurs, trying to hex them and calling them filthy half-breeds and ordering them to unhand her.
In Romance of the Three Kingdoms, Zhou Yu is completely unhinged by the failure of his last plan against Zhuge Liang. This was probably part of Zhuge's plan: Zhou Yu's rage causes a wound to reopen and he sickens and dies shortly thereafter.
The Big Bad of one series of Star Trek tie-in novels starring Q, the Q-level being known only as 0, goes through a few of these. In the series of flashbacks that Q shows Picard, 0 spends decades "testing" the mettle of a powerful civilization known as the T'kon empire. As the T'kon continue to overcome everything 0 throws at them, he gets more and more pissed off, and in a childish fit of pique, triggers a supernova in the middle of their empire, completely wiping them out. In a flashback set not long after that, 0's defeat at the hands of a regretful Q drives him completely insane; he goes from a "charming" rogue to a deranged madman obsessed with revenge and crooning little ditties to himself. His state of mind can best be seen in the following:
0: Q WILL PAY. ALL THE Q WILL PAY. Q AND Q AND Q AND Q AND Q AND QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ...
Must be endemic to villains in Q-related novels. Q-Squared, by Peter David features the return of General Trelane, an antagonist from Star Trek: The Original Series. Trelane manages to set himself up as quite the Magnificent Bastard... until Picard faces him in a duel, and, upon figuring-out that the Captain is only playing with him, he throws a temper tantrum and reveals himself to be the Psychopathic Manchild he really is.
Ahab's final scene, locked in combat with the white whale in Moby-Dick, produces the famous quote: "Towards thee I roll, thou all-destroying but unconquering whale; to the last I grapple with thee; from hell's heart I stab at thee; for hate's sake I spit my last breath at thee!"
In The Color Purple, Albert experiences this after Celie calls him out. He just loses his cool, stammers incoherently, and suffers a mental collapse. Ultimately, this forces him to change, turning him to a legitimately nice individual.
In the Outlander series, Captain John Randall normally, in allhisvillainy, has two main expressions: self-assured calm, and mild irritation. The first time he's shown to completely break down sobbing is when he confesses to Jamie that he loves him, and goes completely psycho when Jamie refuses to respond in the affirmative when he orders Jamie to tell him that he loves him too.
Hatsumoto from Memoirs of a Geisha already started losing it when she found out Mameha had taken Sayuri as her little sister and training her to become a geisha, trying several tactics to beat Sayuri, showing just how desperate she was, but Mameha would find ways to best her instead. Once Sayuri became a much more successful geisha than her at such a young age, Hatsumoto really does lose it. And Mameha had no trouble deliberately driving her further off the edge. Which she would eventually succeed.
Animorphs largely averts this by having most of the villains start off Pre-broken-down. However, when David finds out he has been deceived and trapped, he begins sliding into this, and when he finds out that his fate is to be trapped inthe body of a rat for the rest of his life, he slides over the scale. All through the two hour morph limit, and up until they transport him out to a small island and leave him, he never stops screaming the word "No" over and over, even after they leave. Understandable, considering the circumstances. Even Rachel is haunted by that experience throughout the series.
Big Bad Visser Three (who becomes Visser One near the end) being one of the already broken down villains, has a reverse breakdown when he is finally defeated. All the series he's been an Ax-Crazy personification of pure rage, but after his defeat, for the first time in the series he is calm and soft spoken. He simply slumps down and quietly accepts that the Animorphs won.
Peter Teleborian, a Smug Snake who physically and emotionally abuses the children placed under his care suffers this big time in the climax of The Girl Who Kicked the Hornet's Nest. As his testimony is ripped to shreds by the lawyer of of one of his former victims he looses his cool and can only stammer; when the cops come to take him away for possessing child porn, he can't even bring himself to speak he's so shocked.
In John Gardner's Grendel, the eponymous monster who has been philosophical and introspective while he terrorizes the Danes throughout the book has a breakdown when he finally meets someone stronger in Beowulf. Grendel is reduced to blindly running away while crying for his mother after Beowulf tears off his arm.
Sisterhood Series by Fern Michaels: In books like Sweet Revenge, Lethal Justice, Free Fall, and Hide And Seek, the Vigilantes will drive the Big Bad into this, and then go directly to them to administer justice face-to-face.
Sweet Revenge has Rosemary Hershey suffer one spanning almost the entire book. First she finds out that Isabelle Flanders, the woman she framed and ruined, is back in action and is preparing to sue Rosemary. Then Rosemary gets more than one envelope containing pictures of the three people she killed, and images of her at the trial, with words like "thief" and "liar" scrawled on them. She is told by her husband Bobby Harcourt that he wants a divorce. She finds out from her horoscopes (she puts a lot of stock in them, apparently) that bad things are going to happen to her in the near future. She gets an original set of blueprints made by Isabelle, which Rosemary thought she destroyed, and she wrecks her bathroom in a frantic attempt to destroy it. She gets questioned by reporters about the trial. The big important men she slept with are questioned by said reporters, and these men bombard her with phone calls. She ends up firing all her employees when they can't come up with a good set of blueprints. She seems to regain her composure when she gets a good set of blueprints delivered to her. However, when she and Isabelle Flanders go to an event in which their architectural blueprints will be judged, Myra comes up and reveals to the entire architectural board that Rosemary and Isabelle's blueprints are exactly the same, with their signatures being the only difference, and that as far as it can be determined, Rosemary plagiarized Isabelle's work! By the time the Vigilantes go to Rosemary and threaten to bury her alive if she does not confess to her misdeeds, Rosemary has lost it. Oh, and it doesn't need to be said that Rosemary went to an institution and may never become lucid again by the end of the book.
James Taggart suffers a truly devastating Villainous Breakdown at the conclusion of Atlas Shrugged; when he realizes that he would rather kill John Galt than survive himself, he suffers Mind Existence Failure and enters what is implied to be irreversible catatonia. He exits from the plot forthwith.
Septimus Heap: DomDaniel suffers a Villainous Breakdown including begging for mercy when he's spotted by Marcia Overstrand as having tries to use his Apprentice's body as his own to fool them.
"Rumpelstiltskin": When the queen reveals that she knows Rumpelstiltskin's name, foiling his plot to steal her child, his reactions vary depending on the edition. The early editions simply have him leaving in a huff. The later ones have him going so insane with rage that he tears himself in half.
The Tamuli, by David Eddings, has Zalasta. He starts as The Chessmaster to such a degree that a total list of his manipulations takes twenty pages and covers centuries of history. The gradual loss of everything he cared about, his growing contempt for his son, coming into too-close contact with an Eldritch Abomination, trying and failing to kill Sephrenia, and his plans coming crashing down around his ears multiple times, all contribute to making him a furious psychotic avenger by the end of the series.
Nicomo Cosca in Red Country. He doesn't take at all well the loss of his gold. Specially shocking given his usual Affably EvilLovable Traitor personality.
In the climax of Anno Dracula, Genevieve and Charles gain an audience with Dracula. He acts welcoming but arrogantly self-assured in his power, boasting to Genevieve about how he has created a utopia for the undead. However, once Charles assassinates Queen Victoria, thus dissolving Dracula's claim to the throne, he devolves into a feral beast, "[spitting] rage and hate."
Countess Highglider in Captive of the Red Vixen becomes explosively unhinged towards the end, eventually ranting that the Council of Countesses are commoner sympathizers and how she wants her son back, in front of the entire Council, who unanimously vote to have her committed to the same asylum where her son was incarcerated.
In the Relativity story "Master Blankard's Pawn", Blankard's henchman Rasmas has spent the entire story setting up an elaborate deathtrap for the heroes, only to spring it while standing inside it with them.
Rasmas: The charges have been placed. The timers have been set. There’s no way to undo any of this! (grabs Overcast by the lapels) They’ll never find our bodies!
In Words Of Radiance, Szeth suffers an absolutely epic one when he discovers that the Knights Radiant actually are returning, which means he is not Truthless, which means that he could have refused his masters' orders, which means that the literally millions of deaths on his hands are absolutely and unequivocally his fault.