Angel: The Rogue Power Jasmine flips out after the heroes break her power over humanity making everyone sees her as a hideous monster. She tries her very hardest to kill Angel, all the while ranting and screaming about how she sacrificed so much to offer humanity peace and love. Now she only wants to kill all humans after being rejected. In the end she's reduced to begging Connor for help. The disillusioned Connor finishes her off instead.
Ashes to Ashes: In the final episode, Magnificent Bastard / Trickster DCI Jim Keats has a spectacular one when he loses his patience with the people he's trying to turn against Gene Hunt, beats Gene to a pulp, and breaks the illusion of CID to expose the true nature of Purgatory. It seems like he's going to win, but Alex fixes the world, the team rallies together, and Gene helps them all cross over. He then has another, less badass one when he breaks down upon being beaten by Gene, to the point of laughing in a deranged fashion, speaking in random, babyish sentences and stumbling off in no particular direction.
Breaking Bad: Though it's arguable who's the good guy in the series, Hank Schrader gets one of these. After he discovers that news of his wife being in a car crash was faked (to allow Walt and Jesse to escape an RV they were hiding in), he tracks down Jesse at his house and assaults him, outraged that they knew some of his important personal details. He ends up being thrown out of the DEA as a result.
Walter has a rather terrifying breakdown at the end of Crawl Space, complete with insane laughter.
Buffy the Vampire Slayer: Season 3: The Mayor, upon seeing Faith in a coma, shows rage for the first time and tries to smother Buffy in her sleep and attacks Angel in front of civilians, going so far as to break his foul language etiquette. He regains his composure for the final showdown though, his last words being "Well, gosh!"
Mayor: Murderous little fiend! Did you see what she did to my Faith?
Angel: Hadn't any plans to weep over that one.
Mayor: Well, I'd get set for some weeping if I were you. I'd get set for a world of pain! Misery loves company, young man, and I'm looking to share that with you and your whore!
A minor example can also be found in the first episode of season 3. "Humans don't fight back! Humans don't fight back! That's not how this works!"
The Master has one in the pilot after he senses that Luke - the key to his plan to free himself - has been killed.
Cold Case: George Marks, the Smug Snake serial killer undergoes this in the season 2 finale. Lilly refuses to let George get to him, and after confronting him with his mother's crime she rips apart his god complex saying that all George is is a scared little boy who's mommy never loved him. George proceeds to loose his cool and screams at Lilly to shut up repeatedly. After watching him walk away in a previous episode, witnessing him loose his cool was kind of satisfying. In the season 1 finale, the villian Jim Larkin keeps his cool until they reveal that they have dna evidence linking him to the crime, at which point he completely drops the facade and screams about how both of his targets were supposed to have died that night. John Smith also experiences this twice; first when his target avoids being broken, and the second when the detectives figure out where his victim is being held in time to save her.
Columbo: In the episode "Dagger of the Mind," Lt. Columbo plants evidence implicating a Shakespearean actor in a murder, causing him to go mad. But it's okay.
Criminal Minds: The episode "Parasite" starts with the villain in the midst of his breakdown, and he just gets worse as it goes on.
In fact they have a Real Life term for this, "devolving," and killers are most dangerous when they start spiraling into this.
Dexter: Has a pretty good one in Episode 9 of Season Three, when he learns that he's been manipulated by Miguel Prado after believing the man to be his friend. It's internal, but still. Another in season 2: "It is OVER WHEN I SAY IT IS!"
Dirty Sexy Money: Simon Elder finally gets control of his rival Trip Darling's business, only for all the major stockholders to refuse to work with him and abandon the company. When one of his employees has trouble turning off the television, playing news of the company's catastrophic stock drop, Simon throws a champagne bottle into the TV. In the middle of a board meeting. Ironically it turns out this wasn't a Batman Gambit on Trip's part as Simon assumed at the time; he had genuinely given up and was just as surprised at the stockholders' actions.
In the season 18 story "Warriors' Gate", the slaver leader Rorvik finds his spaceship trapped in a place between universes. Although Rorvik at first tries to deal with his situation in a rational and methodical manner, over the course of the story he gradually breaks down under stress. By the end of the story, Rorvik is ranting and raving, and he finally kills himself and his crew in a failed attempt to escape by creating a backblast with his ship's engines.
"I'm finally getting something done!"
At the end of "The Last of the Time Lords", a combination of seeing his previously unstoppable universal domination plans crumble into nothing within the space of a few minutes and seeing his old enemy restored to full health (and turned into a glowing omnipotent being at that) is enough to reduce the Master, previously a Magnificent Bastard to rival any, into a hysterical wreck:
The Master: You can't do this. You can't do this! IT'S NOT FAIR!
The ending of "The Sun Makers" has the Collector, upon realizing the revolution has finally caught up with him, reduced to a babbling wreck as he slowly (and literally) goes down the drain.
The Dalek Emperor has one at the end of "The Parting Of The Ways" just before he is destroyed by Rose Tyler.
And then there's Soldeed, who has probably the hammiest breakdown of all:
Soldeed: My dreeeams of CON-QUEST!
Dollhouse: Adelle DeWitt, the Anti-Villain/Anti-Hero (sort of), has a variant in the episode "Stop-Loss," where she begins to drink heavily due to realizing she has no life outside her job and that she's rapidly losing control even of that. Good-guy Boyd snaps her out of it by reminding her of her true nature...but in a Subversion, she only gets more villainous, since she now remembers what a Magnificent Bastard she can be.
And then Double Subverted: she was actually working against the bad guys by putting Echo in the Attic.
ER: A few instances. Chief nurse Eve, though not really a villain (the rest of the ER cast might disagree), loses her ruthless, cool head when she gets dumped from her boyfriend and fired from her job on Christmas Eve. She promptly snapped at Kovac, who had warned her about her impulsiveness for punching out a patient, and Sam, arguably the only person who tried to stick up for her.
Another instance was a former patient Curtis Ames, who had lost the use of his right arm under Kovac's care. He would subsequently be divorced from his wife, separate from his kids, watch his kids call another man "dad" and lose his job. When he brought the kidnapped Kovac to his old house, he started laughing maniacally and mentioning how he shouldn't call the run-down, filthy house "beautiful". And in the scene where there were police lights outside his house, he started yelling at the flashing lights to leave him alone.
Farscape: The Big Bad of a particular season or plot arc will always suffer one of these at some point; Captain Bialar Crais of the first season began cracking up almost immediately after we met him, and went on a Roaring Rampage of Revenge for the accidental death of his brother, which wasn't to end until Peacekeeper High Command removed him and he was forced to become a hero.
Commandant Grayza's was quite severe, given that unlike the other two villains, she had very little to make her likeable or sympathetic (the date-rape had plenty to do with it). After being captured by the Scarrans and seeing all her negotiations for her vaunted alliance fail, she attempted a suicidal charge on the Scarran base that would have resulted in the death of all who were still loyal to her. However, Mauve Shirt Captain Braca decided she'd gone too far, and arrested her, allowing Scorpius to retake the ship.
And just to illustrate how badly she was doing, we had this exchange:
Grayza: All that astonishing wormhole knowledge and still you will not share it with us. You came in here big and bold, dancing on tabletops. And look at you now; begging for scraps. Crichton: I may be jammed, possibly dead. But I am not begging- you can get that fantasy out of your head. Grayza: (Forcefully) In my hands, you can have peace! I can have peace! Crichton: I have been in your hands. There's no peace there... just power. Grayza:You are so self-righteous! I have used all my skills, my resources for one perfect chance at peace! AND BECAUSE OF YOU, IT IS GONE AND I AM— (She stops, almost in tears, trying to steady herself.) Crichton: (Coldly) Frelled? Screwed? Raped? Welcome to the universe, Commandant.
Later in the same episode Scorpius has a brief one when realizing that he is one tiny step away from crippling the Scarrans; a simple forcefield that given time they could easily get past, but there is no time to do anything other but uselessly shoot at it. He even points a gun at the rest of the group for pointing out that they have to leave.
YoSaffBridg: Durren, you have no idea what he's forced me to... Durren: Yolanda... stop. Just... stop. [Gives her a Puppy-Dog Eyes look] YoSaffBridg: [Lowers the gun she'd been pointing at him] Don't look at me like that. [Raises the gun and points it at him angrily] I SAID DON'T LOOK AT ME LIKE THAT!!!
"Objects in Space." Even through most of River's Hannibal Lecture on how sick he really is, Jubal Early stays cool and collected, until he suddenly realizes she's been on his ship the whole time! And she's fiddling with those shiny buttons! Oh Crap! Cue panic attack.
Flash Gordon: Ming gets this quite a bit in Sci-Fi's show. The best is the series finale, when all of the heroes join forces to bring down Ming's regime once and for all. Aura opts to ally herself with her brother and betray her father at least, but still can't resist trying one last time to explain to him how she still loves him despite all he's done and begs him to just surrender. Ming looks moved and acts like he's about to stroke her face. Cue a few scenes later when Flash runs in to find Ming strangling Aura, muttering how he should have killed her as a baby. He then releases her and goes full nutty trying to chop Flash up with a sword.
The Following: Joe Carroll has an epic one in the first-season finale. The cult he invented is basically one big murderous performance art project, and his control freak nature will not permit the project to go any way other than the way he wants it to. When that starts to happen, well..."freakout" doesn't quite do justice to his reaction.
Heroes: Sylar in Volume 4. After stealing a shape shifting power Sylar begins to suffer a severe case of MPD as his body begins shifting and changing against his will, altering his DNA to new and unfamiliar patterns. It's not long before he's imagining that he's talking to his dead mother, shape shifting into her to talk to her and breaking down sobbing about how no one loves him and how alone he is. This only gets worse as he then tries to seduce Claire, believing her to be "Destined" to be his because the two are immortal, while plotting to be president just so he can feel special and loved for a little while.
See also: Samuel Sullivan. The crumbling starts when the chink in his armor (his murder of his brother) is finally revealed before an audience, and he just comes more and more undone as the other carnies — whom he considers his family — desert him, causing him to lose the source of his power, until it finally all falls apart and he collapses. Major points to Robert Knepper.
Samuel: They can't have all left... they're here somewhere... you can't leave me... you COWARDS! Where are ya?? ...Come back here! You're NOTHING without me!! (collapses into Villainous BSOD).
Kamen Rider Den-O: Kai has a slow, series wide one. He gradually goes more and more insane as the series progresses as a result of sending Imagin into the past through himself, destroying his past selves. Being a Singularity Point like Ryotaro, this doesn't kill him, but it gradually tears away his mental state. Even his Dragon points out he's getting worse. Finally, he discovers that Sakari isn't the Junction Point as he believed, he completely loses it. He uses all his remaining memories to grant all his Imagin physical form and create the superpowerful Death Imagin, sending them on an all-out attack on Tokyo. He then proceeds to try and let loose a blast of temporal energy to erase the entire city from time, along with his army!
Kamen Rider Kabuto: Title Rider Tendou annoys the sanity out of Sou Yaguruma (then Kamen Rider TheBee and the respected commander of Zect's elite Red Shirt force Shadow). Though he's not a villain, he is a Knight Templar about Zect's rules, and Kabuto using Zect's Rider technology while not being part of the organization is not smiled upon. Yaguruma, like many a villain has his original motivation give way to "The Hero must pay!" after not too many instances of being defied by him. Mister "Perfect Harmony" finally completely loses it in the middle of a battle, making him ignore the fact that the Worms (the series' token evil monsters/aliens) are massacring his underlings because he's consumed by the desire to defeat Tendou. It leads to his Transformation Trinket rejecting him for good, to him being disgraced in front of his former team (later), and eventually to his return after taking a level in badass... and jerkass, becoming the dark Rider Kick Hopper. (His Villainous Breakdown continues, because even though Kick Hopper and Punch Hopper beat up the good Riders at first, they still kinda don't know what to do. It's kinda like "Okay, we're dark now. So, now what do we do?" and sit around hilariously trying to be cool and... if it was in a school setting, we'd say 'gothy.' It doesn't erase, but underscore, how far he'd fallen.)
In Kamen Rider Dragon Knight, General Xaviax is starting to show signs of this once the Advent Master appears and starts undoing his work by bringing back Ventara's fallen Kamen Riders. All this following the fact that Xaviax's corrupt Kamen Riders from Earth have all been vented. He was all calm and collected but his mask cracked every so often, especially when he was down to one Rider.
Law & Order: Since the series is a crime drama it's full of all manner of people that lose it when they previously think they'd gotten away with a crime only for those dreams to be rudely shut down. Its spin-off series Law & Order: Special Victims Unit has one episode where Stabler and Benson take down an animal trafficker, who ends up being caught in the end when he falls off of a conveyer belt while running from Stabler. His leg is pinned and injured, and he's screaming at Stabler to shoot him because when he gets out he's going to come for him. Elliot scoffs at the prospect of him getting out (in addition to him being part of a known animal trafficking ring, the animals he'd been selling were also being eaten) and he's still screaming at him even as he's on a stretcher being loaded into a police ambulance. One might think that he didn't appreciate the irony of him being nothing but an animal in a cage.
Lost: Near the end of the third season, Ben became increasingly unnerved by the fact that he wasn't recovering from surgery as fast as he should have (people aren't supposed to get sick on the island — or something like that), as well as his people becoming more and more drawn to Locke. He finally snaps in his spotlight episode, shooting Locke and returning to camp acting quite erratic. Oddly enough, even though things have gotten a lot worse for him since then, he's managed to keep his cool.
He snapped for real when Keamy killed his adopted daughter Alex in season four.
And once again in "The Incident" thanks to Jacob's enemy, who manipulates him into killing Jacob.
The Man in Black tends to be pretty calm and smug, but he goes a little crazy when he sees Jacob's ghost in the jungle. He chases him frantically, and the ancient invincible entity of destruction trips on a branch. He has a brief one when he realizes he's become mortal in the series finale.
Malcolm in the Middle: Lois' horrible mother Ida tries to con a rich man into marrying her by keeping him drugged while they were dating. However, during the wedding ceremony, he starts to come to and, since Ida doesn't have any pills left, she's reduced to clinging to his leg begging him not to go. The scene ends with her sobbing on the floor like a child throwing a tantrum, screaming about how unfair it is.
Merlin: Morgana spends the entire series slipping down the slope of madness, but in the Grand Finale she loses the two things that kept her even remotely grounded, Mordred and Aithusa, her army is defeated, and all she has left is an obsession to kill Arthur. By the time Merlin catches up to her and sticks Excalibur in her gut, you'd be forgiven for thinking she'd let him do it.
With the series proper, Dr. Forrester has one when Joel escapes, quickly recomposing himself when Mike ends up walking up to them. Pearl suffers one when she accidentally activates the return to Earth mechanism on the Satellite of Love.
The Prisoner: In the episode "Hammer into Anvil", No. 6 makes the new No. 2 have one of these, convincing him there is a plot against him with some irregular acts and fake messages, ultimately ending with No. 2 getting rid of everyone, accusing them of being traitors, before collapsing into tears; No. 6 convinces him to resign.
The normally flat, emotionless Captain Neville eventually resorts to slapping Danny upside the head to keep him quiet. No telling if he'll get worse as the series progresses.
Averted Trope with the preternaturally calm Monroe so far...not that it makes the viewers any less keen to see him get his Bass kicked. Played straight for Monroe in "Nobody's Fault But Mine." When Miles and Monroe finally confront each other, Monroe has the opportunity to kill Miles. Instead, he drops his gun and literally begs Miles to come back into the Militia, saying that he needs him. Miles just gives him "The Reason You Suck" Speech, at which point Monroe loses it completely.
The Scarlet Pimpernel: Robespierre in the BBC adaptation is almost always chillingly calm and formal, even if he's ordering mass executions or plotting to violently crush insurrections. Merely getting flustered and impatient is a sign that he's about to retaliate with drastically excessive force, and when the Pimpernel's antics finally make him lose his shit you know everyone's gonna be in for a rough time.
Scrubs: Dr. Moyer in the episode "My Own American Girl", who's refusal to let the main characters perform an urgent CAT scan at an ungodly hour in the morning turns into a temper tantrum that would make a three-year-old embarrassed.
Dr. Moyer: You called me in from home to do an abdominal CAT scan that could wait until Monday morning? Well guess what? It's not happening.
Sherlock: The villain, Jim Moriarty, has sudden mood swings that end with screaming, which he quickly folds back into his smooth, campy personality. These are often triggered by exasperation or frustration.
Sherlock Holmes: People have died.
Moriarty: That's what people DO!
Smallville: MajorZod was never exactly stable, what with being an Axe CrazyLarge Ham with a Hair-Trigger Temper. He spent most of the season slowly deconstructing, as stress and his inability to cope with his failures enroached on his sanity. He was able to keep in under control for most of Season 9 however, recovering whenever he slipped up. In the season finale, "Savior", however, he lost it, following his army's defection. He pulls Blue K knife out from under his coat, jumps on Clark and engages him in a Knife Fight, ranting at the top of his lungs the entire time.
Spartacus: Blood and Sand: Batiatus has these with increasing frequency as the series goes on. His wife Lucretia and resident Rich Bitch Ilithya also get in on the act at least once each.
Star Trek: Deep Space Nine: The day started so well for Gul Dukat in the episode "Sacrifice of Angels": After months of work he finally destroyed the minefield in front of the wormhole, which allows him to recieve massive reinforcements that will turn the slow going war against the Federation and the Klingons into a very one-sided Curb-Stomp Battle. But then the wormhole opens and not a single of his ships comes out, the entire fleet apparently vanished from existence and his expression rightfully turns into a full blown BSOD. Then his highly beloved daughter tells him she won't come with him when his remaining troops have to retreat and that she helped the saboteurs to stop his plans. Then his second in command Damar shots her in the heart because she's a traitor and Dukat slips completely into insanity, oblivious of whats going on around him.
To say nothing of when he freaks out and decides to destroy Bajor: "I'm so glad we had this time together, Benjamin. Because we won't be seeing each other for a while. I have unfinished business on Bajor! They thought I was their enemy! They don't know what it is to be my enemy, but they will! From this day forward, Bajor is dead. All of Bajor! And this time, even their Emissary won't be able to save them!"
Star Trek: The Next Generation: In the episode "The Drumhead," the Enterprise is host to a series of ever-more-paranoid hearings and trials searching for a saboteur or spy. The spearhead of these trials, a respected jurist who is the daughter of an even-more-respected jurist, is pushing for Starfleet-wide witch hunts, all looking for supposed traitors. When she calls Picard to the stand he opens by quoting her father, pointing out that these accusations without cause are destroying the Federation that she claims to love more assuredly than if there actually was a traitor. She begins ranting, damning Picard for having the gall to quote her father, and displays her paranoia to such an extent that the head of Starfleet Intelligence, who had been there to observe, actually leaves the room. When the guy whose job it is to be paranoid thinks you are going to far, you know you've broken.
Star Trek: The Original Series: Khan Noonien Singh in the episode when the crew refuses to bow down to him even as he's slowly killing Captain Kirk in the decompression chamber, even prompting him to yell: "It's so USELESS!" to nobody in particular.
Doctor Janice Lester grows increasingly unhinged and insane while in Kirk's body, her irrational behaviour causing the rest of the crew to be suspicious. By the time she returned back to her own body, she was sobbing hysterically and screaming about how she wanted Kirk dead.
In "The Conscience of the King", actor Karidian's daughter Lenore after revealing she had killed seven of the nine witnesses who knew her father as Kodos the Executioner ran out to the stage with a phraser, trying to kill Kirk, one of the witnesses. The mad glint in her eyes told us that she lost her mind. And when she accidentally killed her father, she broke down into tears and later on, she insistently believed that her father was still alive and still performing.
General Trelane at first tries to pass himself off as A Man Of Wealth And Taste; his true Psychopathic Manchild colours to bleed through around the time that he starts hunting Captain Kirk for sport, but when he posied, ready for the kill, and parents show-up to give him a stern lecture about interfering with primitive species (and presumably, take him to his room), he starts whining like a particularly pathetic five year old.
Tales from the Crypt: "The Man Who Was Death", the Vigilante Man executioner protagonist is reduced to a cowardly wreck begging for his life when he is finally caught and gets the electric chair. This just after he spent the entire episode extolling the virtues of capital punishment.
Los titeres: Adriana GodÃ¡n in this Chilean telenovela has a particularly memorable one: her plans wrecked, she regresses to childhood, revealing that she tried and failed to get her father's love, but he wanted a boy, jumps into a pool and starts playing with her old dolls. Apparently, "to comb the doll" is now slang for a breakdown in Chile.
Tokusou Sentai Dekaranger: Agent Aburera starts out as a cool, calculating Man Behind The Man at the beginning, but gets increasingly twitchy as more and more of his deals are scuttled due to the titular team's interference with his clients' schemes. Eventually, he starts gunning for the Rangers himself, first through indirect assaults using hired thugs and attempting to discredit their integrity, then by attacking them himself in a personal confrontation. It's only at the end, after his black market empire is in shambles and he's out of options that he executes his ultimate plan: invading the Rangers' base directly with an army of Mecha-Mooks, taking control of it, trouncing around the city causing massive destruction in its Humongous Mecha mode, and using the mayhem to lure the rest of the SPD fleet to their doom, all while raving about how he will pave the way for a galactic criminal paradise with their ashes. And after that plan fails, his final words are a Hannibal Lecture about how his dreams aren't dead, and that There Is Another that will take his place and avenge him and bring forth his twisted utopia. (We meet that one in Magiranger vs. Dekaranger.)
V-2009: Anna has a positively epic one at the end of the first season after Erica destroys nearly all her soldier eggs culminating in her going "Rocks Fall, Everyone Dies" on the entire planet.
The Walking Dead: Saw the Governor snap and shoot about a dozen of his own people in the latest season finale.
White Collar: Has the controlled, smug Chessmaster Vincent Adler being crushed to the brink of tears when all the treasure he spent his life searching for blew up in front of him.
The Wire: For three seasons, Marlo Stanfield is completely calm and unflappable, even when things aren't going well for him. In the last two episodes, however, he has two villainous breakdowns. After getting thrown in jail, Marlo takes it in stride as just part of the game. Then he learns that his subordinates have been hiding the fact that his nemesis Omar has been calling him out. He flips out, screaming that he has to step up to defend his name. This is the first time he ever raises his voice.
Chris Partlow: It's some bullshit man. You don't need that shit on your mind.
Marlo: What the FUCK you know about what I need on my mind, motherfucker?!! My name was on the street! When we bounce from this shit here, y'all go down on them corners and let them people know, word did not get back to me! Tell em' Marlo step to any motherfucker, Omar, Barksdale, whoever. MY NAME IS MY NAME!!
In the final episode, Marlo seems to have won, taking a plea bargain and earning his freedom with all the money he's accumulated, on the condition that he leaves the game. Almost immediately he realizes that he can't live as a civilian, picking a pointless fight with some gangsters and screaming to an empty street corner that he's still a force to be reckoned with.
Yes Minister/Yes, Prime Minister: Although not exactly a villain (more of a neutral, amoral antagonist), Sir Humphrey Appleby is usually smug, cool and collected. When he's in charge, that is, which is most of the time. However, when events occur that he did not expect, or someone actually manages to get one over him, his typical response is spluttering, panicked incoherence (not helped by his Sesquipedalian Loquaciousness). See 'The Skeleton in the Cupboard', where an old cock-up he made comes back to haunt him. Or 'The Key', where a threat to his job (and power-base), coupled with being deprived automatic access to the Prime Minister, leads to him desperately climbing out of his window and up a drainpipe in order to see him.
Hacker: We will still be able to destroy Moscow, Kiev and Leningrad...