During the Trial To End All Days, AJ of AJCO was confident enough in her victory that she went so far as to insult both the prosecutor and the judge. However, when it became clear that she was going to be pronounced guilty, she revealed that she had planted nukes beneath the courtroom, fled the courts and activated the world-breaking device that then launched them all into a new world. Breaking down with style.
In Dr. Horrible's Sing-Along Blog, the main character is shown as a Technical Pacifist and Ineffectual Sympathetic Villain who states that killing isn't his style. However once his Jerk Jock archnemesis Captain Hammer states that he's going to steal the girl of his dreams just to torture him, he then goes into a Villain Song where he swears to kill him. This is followed in rapid succession by Captain Hammer's own breakdown when he is made to feel pain for the first time in his life (it's hard to call it a HeroicBSOD when he's shown to be just as evil as the protagonist), and a second, deeper one from Dr. Horrible when his victory coincides with the death of his Love Interest.
Metal Cooler has this in Cooler 2 not just from Goku and Vegeta overpowering the Big Gete Star's systems after they were thought to have been drained dry, but for Goku and the gang constantly pointing out he's ripping off everything Frieza did.
The Evil Overlord List refers to this a few times, most famously #24: "I will maintain a realistic assessment of my strengths and weaknesses. Even though this takes some of the fun out of the job, at least I will never utter the line "No, This Cannot Be! I AM INVINCIBLE!!!" (After that, death is usually instantaneous.)"
No... No... No, no, no, no, NO! MOTHERFUCKER!!! NO!! NO! NO! NO! BULLSHIT!! BULLSHIT!! BULLSHIT!! Okay, okay, okay. Calm down, Dio. Calm down. Calm down.
Terrence in KateModern — goes from smug, quietly intimidating badass in series 1 to raving, giggling psychopath by the end of "Precious Blood".
The Shadow and Rupert van Helden have also suffered this by The Last Work.
In Lost Boys Of The Cascades, Barry suffers a villainous breakdown after his attempt to frame the Klamath Indians for murdering Jennie, Troy, and Dylan falls apart due to Troy surviving and telling the truth.
In The Spoony Experiment review of Street Fighter 2010, Doctor Insano suffers a breakdown after failing to create the ultimate being (instead producing a cute pink blob thing). He begins crying about how his science never works and nobody loves him before his new "son" cheers him up.
"You... I have to admit, you're not the first guy on this island to seriously piss me off, but if there was a competition going on you'd of just won first fucking prize."
In To Boldly Flee, Mechakara trembles and screams in incoherent rage when he finds out Terl tricked him into working for him.
And while we're on the subject of the Nostalgia Critic, in his review ofThe Haunting (1999) he impersonates Jan De Bont as an ignorant idiot who considers himself and his remake superior to Robert Wise and his original. At the end though, Jan breaks down trying to get people to like his childish and hammy edits to the original ending:
Jan De Bont: It's so ingenious! It's so inspired! Hey, I don't care if it was laughed off of The Disney Afternoon! I'M THE ADULT! I'M THE ADULT! I WRITE ADULT THINGS!! (points to the original summary) That...that's stupid! Stupid psychological issues! (spits on and then defecates on it) YES! I'M THE ADULT! I WRITE ADULT THINGS!! YOU'RE DUMB! YOUR DUMB! THIS IS THE REAL STUFF!!! THIS IS THE REAL SCARY STUFF!! ROBERT WISE IS STUPID!!! HE DOESN'T KNOW WHAT HE'S DOING!!! I DON'T CARE IF HE MADE SOME OF THE MOST LIKABLE MOVIES OF ALL TIME!! HE'S DUMB!!! HE'S A DOODOOHEAD!!! IF YOU TOOK POO AND ADDED EVEN MORE POO, THAT'D BE ROBERT WISE!!! HE'S DUMB!!! HE SO DUMB YOU HAVE NO IDEA AT AAAAAAALLLLL!!! LOVE ME!!!! LOVE ME AND DON'T LOVE THAT!!!! THIS IS STUPID!!! lOVE ME!!!! MELOVEMELOVEMELOVEMELOVEME!!!! I'M THE GENIUS!!! I'M THE GENIUS!!! I'M THE GROUNDBREAKING GENIUS!!! ALALA!!! ALALALALALA!!!
In Tribe Twelve The Observer has a glorious one (complete with an incoherent roar that has the words 'SHUT THE FUCK UP!) when Firebrand prevents him from abducting Noah.
When Jotaro is about beat Steely Dan down in Vaguely Recalling JoJo, he becomes very desperate and reenacts Light Yagami's breakdown. It gets worse after Captain Tenille refuses to help Steely Dan.
The fourth Season of Noob shows the toll that the Mistaken for Badass situation between Dark Avenger and Sparadrap has taken on Dark Avenger, causing him to outright quit the game over the situation by the Season's third episode.
The killer in Season Two of Where The Bears Are has one when Reggie takes the call from the police and discovers that his alibi has been faked.
C.T.or rather, her Insurrectionist lover, who took her identity after she died, goes from calm and collected secret keeper, to a screaming maniac when Tucker shows up to rescue Sarge, Grif and Caboose. Keep in mind that he kept his cool while facing down both Tex and Carolina.
In the penultimate episode of Season 12, Felix has been doing what he usually does in his fight against Tucker: Evil Gloating with the usual sociopathic cheer to his demeanor, yammering on in ways that describe his & his partner's plans (which the heroes already know at this point). Ever since his Evil All Along reveal mid-season, he's been cocky and insulting and horribly cruel and just won't shut up. But then, Tucker reveals that Church is inside his armor, using Tucker's helmet-cam to record the entire spiel, and then he sends it to play at the planet Chorus' capital, exposing Felix & Locus' plot to the Federal Army and New Republic that they've been manipulating for so long, bringing the war - which F & L have spent years carefully orchestrating - to a screeching halt. His response? A long stretch of stunned silence, followed by:
"You... You're DEAD!!"
From the same episode, the partner in question goes through a much more subtle one. Locus spends his battle against Washington cloaked and taking opportunistic shots at him. But then Washington gives a speech that deconstructs Locus' Consummate Professional image, culminating in him calling Locus a coward. This pisses off Locus enough that he abandons tactics and goes for a straightforward beatdown of Washington. When the scene cuts to him again, he's seen hesitant for the first time in the series, only telling his mooks to follow protocol.
This Gmod video has a non-verbal example where Painis Cupcake imitation Ass Pancakes becomes visibly unhinged over his inability to bonk Derpy Sonic's skull.