The tendency of relatively calm and composed villains to lose their cool over the course of the story. Usually happens to arrogant Chessmaster-type villains (especially if said villain is a Smug Snake), as control of the situation slips from his or her grasp and things come up that they Didn't See Coming. Most notable when it happens towards the end of the story to show how pathetic the Big Bad is when things are not going exactly as planned ("This Cannot Be!"). Especially apparent if they previously had Creepy Monotone or Dissonant Serenity, and suddenly start Chewing the Scenery (when they do this because their plan is succeeding, it's more of a case of Drunk on the Dark Side). Thus, a Villainous Breakdown is frequently a trigger for a One-Winged Angel transformation.
Similar to Oh Crap, though the difference lies in that Oh Crap moments involve a single moment where the character finds that they are screwed, while Villainous Breakdowns have them see it coming from miles away. The good-guy counterpart is the Heroic BSOD.
Note that this isn't necessarily an example of Diminishing Villain Threat. On the contrary, some villains become much more terrifying and dangerous when their façade of cunning and civility crumbles. They might even end up dishing out a vicious No-Holds-Barred Beatdown to the Hero(s), engage in some Cold-Blooded Torture or, goodness forbid, drop the Villain Ball and Just FINISH HIM Already instead of playing around. It could even lead them to stop holding back their true nature and assuming their true form as the monster they actually were. Affably Evil and Faux Affably Evil villains are prone to becoming far more terrifying and disturbing when they undergo one of these because their fairly normal personality serve to emphasize their breakdown.
A somewhat less common, but not exactly infrequent, form of breakdown results in the exact opposite reaction: the villain lapses into a catatonic state as the shock of their defeat robs them of their wits. Another form of breakdown occurs due to Heel Realization causing a Villainous BSOD. Yet another is the impotent flailing of a defeated villain, left with nothing without power.
The scale of the breakdown generally depends on the nature — and overall impressiveness — of the villain. A Smug Snake, being generally inclined to rate their own abilities higher than everyone around them anyway and less equipped to deal with setbacks, will usually break down more frequently and over relatively minor spanners in the works, with the resulting tantrum using being less-than-impressive. A Magnificent Bastard, on the other hand, is already the type of villain who's not inclined to sweat the small stuff, and will usually respond to most setbacks fairly calmly; however, this merely means that if their plans suffer a particularly impressive implosion, then this will usually make their resulting breakdown all the more epic. Similarly, a relatively minor or small-scale villain may have a breakdown on the level of a temper tantrum or explosive Cluster F-Bomb rant; a major or larger scale villain, however, will frequently have a breakdown with potentially (and quite literally) world-ending consequences. As the old saying goes, "The bigger you are, the harder you fall".
In another type, the breakdown won't be one, sudden explosion, but a gradual progression of smaller, gradually increasing breakdowns happening over the course of the story. As their Evil Plan comes closer to failing or their plans fail again and again, they simply get worse and worse as the breakdown continues, until finally, they lose it completely. This normally happens in the buildup to the final confrontation. This can overlap with Sanity Slippage, were a character gradually becomes more and more insane as time goes on. And as a special treat, when the villain finally goes so crazy and bat-shit insane that he completely loses it, other characters tend to hit him with a You're Insane!.
Another type of breakdown is when the villain becomes extremely furious when either something happens that isn't part of his plan or his plans end up being ruined, or both. Once pissed off, he may lose his temper and say such phrases as "Why Won't You Die?", "I've had it with you!", "You Bastard!", or even "This Is Unforgivable!". Either way, the villain will scream more and more at the other characters in fury before being bested by them.
For Anti Villains, or ones with a Freudian Excuse, a breakdown can either play Cry For The Devil (if their breakdown is pathetic) or cement them on the far side of the Moral Event Horizon (if they went Jumping Off the Slippery Slope during their breakdown).
This is not the case for all villains, as some can manage to never have this breakdown, which can be played in two ways. Firstly, the villain is a Villain Sue and a Karma Houdini and never suffers a loss; secondly, played more for horror, the villain manages to keep his cool even as he's losing making him/her seem more horrifying and intimidating to the heroes.
The Example above is Azula from Avatar The Last Airbender and is one of the best examples of this trope. She becomes progressively more unhinged throughout the Grand Finale. kicked off by the betrayal of Mai and Ty Lee. She really starts going nuts when her father Ozai promotes her to Fire Lord, only to name himself the Phoenix King, effectively rendering her a Puppet Queen, goes even farther when she fires her servants out of paranoia when she thinks they are plotting against her and she hallucinates a conversation with her mother, and climaxes after Zuko and Katara interrupt her coronation and defeat her in an Agni Kai. Her last scene is of her weeping, screaming, and writhing on the ground; chained, and pathetic. Word Of God has stated that she's now in the nuthouse and is supervised around the clock (not that it makes the breakdown any less disturbing to watch).
a much lesser Example is Ozai At the start of his battle with Aang, he's smug and confident. But after Aang finally achieves the Avatar State and proceeds to kick his ass all over the place, Ozai is visibly terrified and tries to FLEE. After Aang strips him of his powers , Ozai is simply a wretched shell who speaks in a monotone, "don't give a shit about life anymore" voice.
another good Example is Zant goes through a rather sudden breakdown when you battle him in The Legend Of Zelda Twilight Princess, changing from a very calm Evil Overlord to a spastic, shrieking mess in the blink of an eye. He went through something similar in the game's backstory, after being denied the throne to the Twili Kingdom, which may explain the sudden breakdown when you face him; it was always there and he was just holding it in (which wouldn't be that hard to do, since everything was going his way and he had no need for a breakdown...until Link and Midna storm his palace and break down his front door, that is.) His breakdown can be observed during the battle itself [link]
. His fighting style (when he's not mimicking other bosses) is already rather crude and childish, but it grows more and more frantic each time he gets hit. By the end of the fight, his Teleport Spam and sword swings are wildly unpredictable.
another is Cesare Borgia from Assassins Creed Brotherhood have a essentially one long breakdown/Humiliation CongaHe loses all his supporters, kills his father, the Pope, and is betrayed by his sister, causing him to lose the Apple of Eden to Ezio Auditore, the mastermind behind his family's decline. Then the remnants of his army are destroyed with the Apple-wielding Ezio and his fellow Assassins, while the Cardinals - whose appointments Cesare had paid for - turn their backs on him, backing a family rival to succeed his father as Pope. He's finally driven out of Rome by Ezio's supporters and arrested by a general whose family he'd pressed into service, then dragged away by his former elites — they're the Papal Guards after all. He escaped a year later and made an attempt at a comeback in Spain, under the command of a relative in France, but Ezio followed him — thanks to the Apple — and fights through his entire army alone, then manages to finally defeat Cesare in personal combat. At this point, all he can do is rant and rave about how no man can kill him, so Ezio leaves him in the hands of Fate...by dropping him off the castle wall.
Even his would-be Badass Boast of "If I want to live, I live. If I want to take, I take! If I want you to die... you DIIIIIIE!" counts, since it's declared when Lucrezia reveals that their father had poisoned him, this being after his father talked down to him for starting the whole mess and denied him access to the Apple of Eden or funding to continue his military campaign in Romagna. Depending on how much side content the player has done before this point, you get to see more of just how badly things had fallen apart for the Borgia regime, yet Cesare is just finding this out now.