Jerkass Ball

This is when Took a Level In Jerkass is plot-requested.Sometimes, an otherwise nice person becomes a Jerkass unexpectedly, mostly to advance the plot. May overlap with Nice Job Breaking It, Hero, Depending on the Writer and Comedic Sociopathy.

Anime and Manga

 * In Ranma One Half, Ranma's dad Genma was always a bit of a careless Jerkass, but definitely took a level with the introduction of the Nekoken (a super-secret technique that can only be taught by torturing children), turning him from Bumbling Dad to insane, nightmare, dad. Its also a case of Never Live It Down, since he rarely does anything else that approaches that level of horrific stupidity.

Comic Books

 * Cry for Justice hands the ball to several heroes, making them increasingly violent and sadistic in their efforts to track down criminals. The worst of them is Ray Palmer, who tortures information out of Killer Moth by shrinking to a microscopic size, entering his brain and enlarging slightly to simulate a stroke--the same method Jean Loring used to kill Sue Dibny.

Literature

 * Septimus Heap: The usually nice titular character was rather mean to his sister Jenna at the beginning of Darke, for ostensibly no reason other than to disregard her warnings about a building danger in the Palace.

Web Comics

 * In Gunnerkrigg Court Chapter 31 "Fire Spike", the cool-headed and stoic Annie argues with much-less-cool-headed Reynardine, beginning with copying her friend Kat's homework, effectively leading to revealing to him a secret that her mother has kept from him her entire life, one that Annie had learned about not 10 minutes ago. She harshly hammers the nail into him about it for little reason until he retaliates in frustration by making what is essentially The Reveal to her. Annie generally never acts this way and doesn't have much reason other than being visibly frustrated previously, but as a whole she's generally stoic.

Western Animation

 * This happens on My Little Pony Friendship Is Magic a lot.
 * In "The Ticket Master" Twilight receives a ticket to the Grand Galloping Gala everypony starts to chase her for it including her friends, yes, even Fluttershy.
 * In "Bridle Gossip" when Zecora a zebra comes to town and is the victim of Fantastic Racism by everypony including the Mane 6 minus Twilight and Applebloom. Applebloom later follows Zecora home to see if the racism is justified and the mane 6 try to get her back. Hilarity Ensues
 * in "The Super Speedy Cider Squezzy 6000" Ponyville's insensitivity and impatience almost costs the Apple Family their livelihood.
 * The Apple family ponies were being pretty inconsiderate to their customers also, what with making them stand in line all day only to sell out early and leave the ponies in the back empty handed. Would it have killed them to raise the price? Or set up a pre-order system? Or at least limit the amount of cider per customer per day so Pinkie Pie couldn't walk off with a whole load while the others go thirsty?
 * In "Putting Your Hoof Down" Angel domestically abuses Fluttershy forcing her to go to the market so he can eat. Once there everypony hates her guts. For example a cherry costs her 10 bits but when another pony buys another cherry it costs only 2 bits. Tomatoes are now 2 bits rather than the one they were yesterday.
 * Fluttershy eventually goes mad with her assertive training and starts bullying anyone who so much as vaguely inconveniences her. She snaps out of it when she manages to make her friends cry as a result.
 * Let us not forget "Lesson Zero", where the usually fair natured Twilight Sparkle has an outright Sanity Slippage from being tardy in writing a friendship message, and resorts to making a problem in order to have material.
 * Other Mane 6 bounce with the ball laugh at her problem and she is overreacting.
 * Pretty much every Family Guy character is prone to this based on Rule of Funny. Lois and Brian, who by default are somewhat straight-faced and fairly empathetic characters, can become selfish, vindictive and outright sociopathic jerks if it helps with the shock value comedy. To a lesser extent Seth Mc Farlane's other works are prone to this as well.
 * A lot of Tom and Jerry shorts employ this, having Tom instigate the conflict so that while Jerry dishes out Disproportionate Retribution for the rest of the short the audience still roots for him (for the most part).
 * Combination of this an Idiot Ball in The Dreamstone, Rufus is usually extremely obedient and friendly to the point of being saccharine. Whenever the plot calls for the Urpneys to steal the Dreamstone however, he seems to gain an arrogant streak, disobeying the Dream Maker's orders and messing with the stone so as to give the villains an opening.
 * The heroes occasionally overzealous handling of the Urpneys counts as this as well, especially when using greyer methods of desposing of them such as Heel Face Brainwashing. Granted the Urpneys are villains, if very pathetic ones, but at least compared to the Noops' exaggeratively docile nature otherwise it's a bit of change.