(NOTE: THE TERM "BREAKDOWN" IS AN SHORT VERSION FOR "MENTAL BREAKDOWN")
A Villainous Breakdown is the tendency of relatively calm and composed villains to lose their cool over the course of the story. Usually happens to arrogant and prideful 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 Mr. Big Bad is when things are not going exactly as planned.
Especially apparent if they previously had a monotone voice or serenity as they were active, and suddenly start Becoming as LOUD as the entire cast of the peice of garbage called "Universal soliders". Thus, a Villainous Breakdown is frequently a trigger for a One-Winged Angel transformation.
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) or, goodness forbid, drop their behavior and Just FINISH DESTROY 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.
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 Chessmaster, 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 FOUL LANGUAGE.
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 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 their own mind completely. This normally happens in the buildup to the final confrontation.
If it happens to a Smug Snake or a Complete Monster it could be very triumphant for the Heroes and for the viewer.
Here are some other examples of this trait: