Archetypal Machiavellian Magnificent BastardRichard III goes through a Villainous Breakdown just before the play's final battle, hallucinating that the ghosts of everyone whose deaths he's caused are visiting him and telling him to despair, and ultimately realizing that after everything he's done, the entire world (including himself) hates him utterly.
When King Richard lashes out at a messenger bringing news of his enemy Buckingham... only the message turns out to be that Buckingham's army has been defeated.
The Wizard has a very quiet dignified one of these at the end of Wicked where he learns that the Wicked Witch whom he believes was successfully assassinated on his orders was actually his daughter. He sinks to the ground completely ignoring both his ally and the returned and angry Glinda for a long while.
Elphaba (said Wicked Witch) has a mind-blowing breakdown in the song "No Good Deed", as she thinks her boyfriend Fiyero has been tortured to death while she couldn't do anything to help.
And it happens again at the end of the show when he finds out that the beggar woman he just murdered was his wife Lucy, that Mrs. Lovett knew she was alive the whole time, and that she lied to him. He promptly mourns dramatically, goes fake happy and kills Lovett in a psychotic rage, and then slips in a Heroic BSOD (or Villainous BSOD) as he cradles his wife's lifeless body, not even bothering to do anything as little Tobias comes and slits his throat.
You have murdered me! You have murdered me! Murdered! Murdered! Murdered! Murdered!
Salieri from Mozart L'Opera Rock, notably in the showstopping "Le bien qui fait mal" number, which combines him going insane with jealousy with some intriguingly sadomasochistic lyrics ("it feels so good to sufffer", "real delights require torture", "I love having [this insanity] inside my skin", etc.) He's a special sort of guy.