Mac: I don't like you! Don't you get it? Nobody likes you. You're annoying and weird and you talk too much, and Bloo's name is not Chester, Mr. Herriman is not a badger, that's not how you play checkers, and protein doesn't come from bananas, it comes from nuts, which you don't need to eat more of because you are nuts! You're chock full of nuts! You're so nuts, you drive me nuts! (Jumps in Goo's face) Get it? Get it? Get it? Get it? Get it? Get it? Get it?! Everybody thinks you're a nuisance and they all want you to just GO HOME!
This was justified as Goo was pushing him to his limit and any one else would lose it after a while. Mac only harshly told her to go home after MONTHS of asking nicely and getting kick out from the foster home because of her.
Finally reversed in The Movie, where we find out that Eddy was just acting that way because he idolized his older brother, who we find out is a far bigger bastard.
A large amount of the cast of The Simpsons have evolved this way as the show has leaned into zanier, more satirical, territory. The most infamous case of this occurred with Homer Simpson: While he was always selfish and moronic, he initially had some level of goofiness and kindness to offset this. When Mike Scully took over (in season 9) and shifted the show into more of an overt comedy, his selfishness and sociopathy went to Q-esque levels. When Al Jean took over, he toned down the character, so now he floats between his early era goofiness and his infamous Scully-Era persona.
This is deconstructed in The Simpsons Movie when Homer's impulsive behavior puts the town in danger and Marge leaves him. He does learn his lesson and sets things right at the end though.
Krusty the Clown went from being a slightly weary and cynical old showbiz figure, to a greedy, shameless sellout who doesn't care the slightest if his licensing products are crappy, or even downright dangerous.
Even normally sane characters, such as Marge and Lisa, act rather mean spirited or outright unstable at times.
Milhouse definitely took a level in this, to the point that he ruined Bart's relationship with the Girl of the Week simply because Bart didn't visit him when he was suspended. He also tried to manipulate Lisa during the ending of The Simpsons Movie by saying Colin was dead and his last words were for Milhouse to take care of Lisa and hold his hand.
He also tried to rip off Bart in Bart Sells His Soul. Bart sold him his soul for $5 and Milhouse tried to sell it back for $50, just to rub it in his nose.
Geoff from Total Drama undergoes this in season 2 when, as the host of the aftermath show, he let fame go to his head, to the point where he tried to drop an anvil on his former friend DJ and feed Gwen to a piranha.
The case can also be made for Courtney after her return to the show in season two. She went from being a slightly stuck-up and haughty girl who was still civil enough to be the Team Mom, to a scheming, spoiled princess who uses her lawyers to cheat her way to numerous advantages on the show. She basically transformed from a significantly better version of Heather to a straight up Expy.
One can argue that Beth got this as well during season two, as she tends to get angry a lot more often (compared to Season 1 where she only got mad at Heather), as well as being able to manipulate Courtney, of all people.
In World Tour, both Geoff and Courtney (the latter at least in the first half of the season) are back to form, only for Cody, Gwen, and Duncan to get jerkier.
Subverted in season 5 with Mike when he starts breaking things For the Evulz and laughs when Sam gets stung by a jellyfish. It's actually because his evil Split Personality Mal is re-emerging and taking control of Mike's body. Mike himself remains a Nice Guy who is terrified of what's happening.
It's safe to say that Flanderization has engulfed the cast of Family Guy so intensely that it's easier to list characters who haven't taken a level in jerkass. The best instance of an inversion is Stewie, who Took a Level in Kindness to the point where he's now the most likable character on the show. (Relatively speaking, of course; he used to be evil, and nowadays he's just an asshole.)
Cleveland has had some Jerkass moments since he got his own show.
Connie went from being the standard Alpha Bitch to a downright bitch, which was best demonstrated when she made fun of Meg shortly after the latter returned to school from being in prison for three months (to compensate she pays for this, and a lot of her Jerkass moments, to dire extremes).
Even Brian followed this route to the point where he got told off by Quagmire, of all people.
Brian reached a high level of jerkness in the episode "Brian Writes A Bestseller". After Stewie helps Brian write a crappy novel for the sake of popularity and sales, Brain's ego becomes so inflated that every minor thing that goes wrong or bothers him (being seated next to an Asian person on a plane, being seated in the back of a restaurant, etc) causes him to get pissed off and blame Stewie for it. Brian angrily tries to justify his behavior by saying he deserves the best of everything because he wrote a bestseller. After Brian took a huge crash and burn on live television when it was revealed he is a sham, Brian, during his apology, still blamed Stewie for his failures. Stewie just gives up pointing out the contradiction and says "This is as good as it's going to get, isn't it?"
Icky Vicky: Originally just a mean, bossy babysitter. Now a sadistic, maniacal human monster not above threatening her own parents to keep her true colors a secret.
Timmy's dad surely began showing signs of this as the series went on.
Timmy's mom did, too. The two used to just be overworked. Now they're straight up abusive.
Timmy himself has become more of a heartless asswipe each season. But thankfully not as awful as Norm the Genie's clone Timmy, whose extreme jerkiness causes his fairy god parents to quit their jobs.
Johnny Canmore from Gargoyles is the youngest and most innocent of the Canmore siblings, and the least inclined to take up his family's ancient feud against the Demon and her kind. As the events of "Hunter's Moon" transpire, he becomes a Jerkass and decides to take up the hunt even as his older brother and sister decide to give up on it.
Sentinel Prime of Transformers Animated was always a jerk, but in the third season it seemed that it was all there was to him. "Predacons Rising" has him take it to villainous levels, being ready to kill his former love interest because she's now part organic. Keep in mind that believing she was dead and being angry with Optimus for leaving her behind was the reason for his Jerkassitude when it came to Optimus and his team. He started at merely Good Is Not Nice, but that's fully gone by season three.
While Megatron from Transformers Generation One was never the nicest being in the world (what with being the Big Bad and all), he became far worse when he was upgraded to Galvatron.
Much of Galvatron's Jerkass traits come from being left in a plasma bath that affected his neural circuitry, following his defeat at the hands ofRodimus Prime. He spends more of his time being a Bad Boss, beating up on his own troops, than he does blasting Autobots, to the point where in "Webworld" Cyclonus - his most loyal lieutenant - gets tired enough of the abuse that he tries to have Galvatron "cured" of his insanity. \ It doesn't work.
In the same cartoon, Cliffjumper in "Traitor" spontaneously accused Mirage of treachery without proof.
The normally affable Brawn belittles Perceptor at every turn in "Microbots", until Perceptor saves their hides. It seems being a Jerk Ass at some point is a prerequisite for first-year Autobot mini-vehicles (Gears and Huffer are normally pretty dour), unless you're Bumblebee or the underutilized Windcharger.
The latter has become more openly hostile towards Spongebob and Patrick and has been getting (quite understandable) payback on them in recent episodes.
While not as much as Mr. Krabs, Spongebob can also shift into Jerk Ass territory at times. A good example of SpongeBob being more of a jerk comes in "A Pal for Gary". He gets some freakish pet, which constantly ruins Gary's life. And SpongeBob constantly punishes the poor snail. Even after said snail saved him from the pet's horrific attempt to eat them. Even by the later season's standards, that's moronic of Spongebob, not to mention quite dickish.
There's also "Choir Boys", in which Spongebob seems to be tormenting Squidward deliberately and reveling in his misfortune.
Patrick even more so. The prime example? The episode "Rule Of Dumb".
There's an episode "Yours, Mine, and Mine", where Patrick 'borrows' money from SpongeBob to 'share' a Krabby-meal together. He of course eats the whole thing himself. Mr. Krabs then makes Sponge pay extra for the toy (which is really just a Krabby Patty that Krabs dressed up as a doll. Score one Jerkass point for Mr. K!) Patrick then claims that since he and Spongebob are still 'sharing' that he gets to play with it. Long story short, Pat eventually never lets SpongeBob play with the 'toy' (that he paid for!) and Pat inevitably eats it just so he can't have it. ...After the two eventually make-up, Pat offers to pay for some new toys, by using SpongeBob's wallet!.
"The Card" revolves around Patrick getting a rare Mermaid Man and Barnacle Boy card that SpongeBob is desperate to own. Patrick then proceeds to abuse the card in so many ways (using it as a lockpick, grilling it and eating it in a Krabby Patty, scratching his back with it) you'd think he's actually doing this all just to torture SpongeBob, who gets increasingly stressed and worried about the card. Surprisingly enough, after doing much more dangerous things with it (such as potentially smashing it or setting it on fire), when SpongeBob warns him, Patrick just says, "You can't expect my usual brand of stupidity, SpongeBob. I like to mix it up. Keep you on your toes." And Patrick flimsily makes it up to the Too Dumb to Live SpongeBob by just giving him new cards. Most people would have at least demanded forgiveness for how he freaked out his friend.
While still a rather sympathetic character, Baloo's TaleSpin counterpart is often shown to act more selfish and egotistical than his original The Jungle Book interpretation (albeit Depending on the Writer). In contrast Louie and Shere Khan are slightly less antagonistic.
While Daffy Duck of Looney Tunes fame was hardly an angel to begin with, he arguably was merely a prankster and an attention freak too warped to understand the extent of his antics early on. As time passed Daffy became more stable and ambitious, as well as more embittered and obnoxious in personality, often acting as an antagonist for Bugs Bunny. His cruelty was taken to extremes with his pairings against 'Speedy Gonzales in the De Patie Freleng era.
It's very easy to forget that Peggy Hill was actually nice at the beginning of the show. She didn't start showing her egotism until season 3 and wasn't even a jerk about it until season 4.
KaBlam!: June started out as a prankster with an enormous heart, but season three-ish, she became more of a Jerkass. Season four ended this.
Velma on Scooby-Doo! Mystery Incorporated has been noted to have taken on Jerk Ass traits, but only because she can't get time with Shaggy to herself due to Scooby-Doo himself. She'll be a Jerkass to start then sadly wistful (to her mom: "Am I radioactive?"). In the heat of the "love triangle", Scooby had taken on the Jerkass label as well, being as pushy as Velma in forcing him to "choose". Currently, now that Shaggy chose him, Scooby's remorseful at how sad Velma is over it and is trying to make up, but conversely, Velma's Jerkass level has risen with the rejection, making a reconciliation difficult.
Episode 15 shows that she might be getting over herself a little; she resents Scooby but she draws the line at his getting hurt.
Daphne had one temporarily in Episode 6, becoming a Hex Girl and stating that her new name was Crush. She reverts back to normal at the end.
Also, it's speculated that she might go into this against Velma, due to Velma having not told them a very important secret that was partially the reason why the gang split up.
As it turns out, she doesn't.
Pietro Maximoff on X-Men: Evolution started as a minor antagonist/rival with some Jerkass traits; when season two hit, he was the only one who doesn't become sympathetic & by the end of the series he takes a level in Jerkass by revealing that Mystique had been impersonating the Professor so she could blow up the mansion, something Pietro and the other Brotherhood members not only knew, but likely helped. Then he fucks EVERYONE over by joining the Acolytes, Magneto's new team, since he had been The Mole the whole time. Then when Magneto lets the Brotherhood join him he puts Pietro in charge, who has Toad work as a TV antenna, and Blob make him sandwiches, throwing them back in his face for the tiniest detail until Blob cries.
Next season, Pietro taunts his sister Wanda until she accidentally derails a train, which unintentionally ends up painting them as heroes to the public & rewarded with cash; he then stages several accidents so they can cash in, ultimately framing the X-Men when they catch on of misusing their powers on a rampage, before deciding to derail another train to send it all home. When he finds out that this time many more people are in danger, he decides to leave quickly out of fear and selfishness, knowing full well that many people are going to die because of his own selfishness. And this is after he was an accomplice in his own sister's Mind Rape and persuaded Toad not to reveal the truth to her.
Irony is he's a Draco in Leather Pants, so the fangirls who actually build up the X-Men Evolution's Fan Dumb, interprets him as the only good member of the Brotherhood, despite the fact that, arguably, he's the most self-centred character the series. Apocalypse is a Knight Templar who wishes for the world to become a Mutant utopia, Magneto is a Well-Intentioned Extremist who wants to avoid another Holocaust, every other Brotherhood member has redeeming qualities (especially Avalanche), Colossus is an Anti-Villain under Blackmail, Gambit is a hired Merc/thief who had some Pet the Dog Moments, Sabertooth and Pyro are just plain crazy... but Pietro is a calculating, selfish dick who serves only himself (and his father isn't a sufficient excuse for some of the shit he pulls).
On Jimmy Two-Shoes, Dr. Scientist was more-or-less neutral in the first season. However, in "Heads Will Roll" he becomes a full blown villain, tormenting Heloise. This could just be the result of not having a major role before then.
Samy Garvin did this twice in season 2. The first time was giving a trophy to Lucius and stating the Jimmy dances like there a lobster in his pants. The second time was intruding on a chat that Jimmy had with Lucius's dad to read a scroll.
Charmcaster in Ben 10 used to treat her golems (or whatever they were) like they were pets. In Ben 10: Alien Force, she can't go a few minutes without insulting the giant ones she has now for everything they do. Also, in the original series, while a troublemaking bitch, she wasn't murderous or out to take over the world. In Alien Force, she does try to conquer the world, and expresses a gleeful desire to kill Gwen.
Ben himself takes a massive level in Jerkass in the third season of Alien Force.
And let's not forget about Gwen, who, during the first seasons of Alien Force, was portrayed as a more calm and responsible person who was less of a Deadpan Snarker, but sometimes prone to Tsundere outbursts. By the third season, she was more short-tempered and snarky again.
Also Mr. Baumann, when he first appeared in Ultimate Alien, he was nice and friendly towards Ben and company. By Omniverse, he seems much more hostile towards him, justified by Ben frequently inadvertently causing the destruction of his store.
Out of all the people in South Park, it's the usually calm and understanding Mr. Mackey who has done this recently, case in point, "Royal Pudding". All he does is scream and cuss at kindergarten students and later, resident Designated Monkey Kyle, for screwing up his play on tooth decay, even though the kids didn't mean to do it.
Throughout most of his Season Fifteen appearances for that matter, Mr. Mackey has started to reveal more and more darker personal issues, albeit ones he frequently vents out on the school's students. His hoarding problem led him to be violently protective of his belongings, while he and the school faculty became so obsessed with avoiding controversy over school deaths they decided to kill Eric Cartman bythrowing him under a bus.
There's also Gerald Broflovski, Kyle's father. For 9 seasons, he's a decent parent, even though A) he's an Amoral Attorney, and B) he's as clueless as every other adult in town. And then there's the episodes "Smug Alert" and "Major Boobage". In the former, he doesn't care what anybody says or thinks after buying a Hybrid, his own family included. In the latter, he becomes almost as bad as his wife Sheila in The Movie, bans cats due to their urine being a drug for "cheesing", and accuses Kyle of being a drug hoarder, which he clearly isn't.
Randy Marsh started off as a somewhat incompetent but well meaning parent, and to an extent one of the more rational adult residents. As his role elevated, he slowly evolved into a sociopathic Man Child whose selfish antics his family often have to face the brunt of (especially his son Stan).
On Brandy & Mr. Whiskers, Mr. Whiskers has his fair share of Jerkass moments, but the one where he really crosses the line is making Ed cry in the episode "Monster of My Skin" after Ed tries to cure his acne:
Mr. Whiskers: I don't want any more of your stupid food! I don't want any more of your worthless cures! If you can't help me, I don't ever wanna see your ugly face ever again. I HATE YOU! I HATE YOU! I HATE YOU!
Both Tom and Jerry seemed to have done this when Chuck Jones started directing the shorts. Jerry would sometimes start their Escalating War without provocation—in "The Year of the Mouse" he and another mouse took on a role similar to Hubie and Bertie, driving Tom insane purely for kicks. On the other hand, Tom's torture of Jerry was severely stepped up, particularly in "Of Feline Bondage", where the sadistic glee he took in tormenting him was rather... unsettling.
In the same episode, Angel the Bunny, always a little bit of a brat, has become violently abusive and borderline sociopathic, making unreasonable demands regarding his breakfast, and then throwing Fluttershy's best effort away for being short one cherry and kicking her out of the house. The closest he comes to any kind of apology is an expression of gratitude after he realises he enjoys Fluttershy's cucumber slices, but then, it was only because of the Stare that he was willing to try them in the first place.
Something like half the town Took a Level in Jerkass in that episode. Some viewers have claimed Gilda had crossed the Moral Event Horizon when she made Fluttershy cry. She arguably did less to abuse Fluttershy than what most of the background characters did in that episode.
This tendency is common with most of the characters in the show, due to its tendency to deconstruct the character's flaws as part of the episode's Aesop. Usually one of the characters will become highly egotistical, neurotic or vindictive (or in some exaggerated cases, borderline insane) for a brief point Once per Episode before learning their lesson and reverting back. Rainbow Dash may be a slightly more consistent example, if only because she has gotten the lion's share of Aesops in recent episodes and thus required more frequently to act up.
Rainbow's Dash's Level In Jerkass deconstructs this, since it led to several repercussions and Aesops that stuck to her character and led to her having Took a Level in Kindness by Season Three.
The second season finale, "A Canterlot Wedding", actually plays with this trope. When "Princess Cadence" shows up, she is apparently an in-universe example, since although Twilight knew her as the best filly-sitter ever, she treats all the wedding preparations and other characters like dirt. However, it turns out to be a subversion. Twilight only later finds out that the real Princess Cadence, who remains one of the nicest alicorns around, has been imprisoned in the mines beneath Canterlot - and that Jerk Ass!Cadence is actually an impostor.
This is a common complaint for Pinkie Pie in the Season 4 episode "Filli Vanilli". In Season 1, she was shown to be extremely protective of Fluttershy, to the point where she refuses to prank her when she and Rainbow Dash are pulling pranks on the other ponies. Here, she says very insensitive things to the pegasus such as "You kinda sounded like a dude".
In Transformers Prime, when Ratchet takes some Psycho Serum, it impairs his judgement, causing him to become increasingly obnoxious and aggressive, even to his own team. Not to mention becoming overconfident to take on Megatron and the Decepticons on his own (which would cost him). He not only flirts with Arcee but he also directly calls out Optimus for being too soft for what had happened to Cliffjumper and also passing several chances to finish Megatron off. While his judgement was impaired, it's implied that he's saying what he would have normally held back. Fortunately, by the end of the episode, he returns back to normal.
In BIONICLE 2: Legends of Metru Nui, Matau's just into the whole hero deal for sheer excitement, meanwhile Onewa seems to take every opportunity to belittle his companions, especially their leader Vakama. In the following movie, Web of Shadows, Onewa's demoted to an even more minor extra, and Matau takes over his role of a Jerkass, putting such a stress on Vakama with his constant complaining that it leads him to do a Face-Heel Turn. Matau makes up for it in the end by realizing how mean he was and bringing Vakama back to their side.
Cecil Turtle in The Looney Tunes Show episode "Customer Service". Originally, he never antagonized Bugs and was instead antagonized by the latter, which led to the turtle winning. Here, he takes away Bugs's cable and keeps trolling him when the rabbit asks for his cable to be restored, all while taking pleasure in it. As a result, he gets outsmarted by Bugs in the end.
Which, ironically, is the opposite of what happened in Cecil's original appearances in the old shorts, where Bugs, usually a Karmic Trickster, was the one who took a level in jerkass, resulting in him antagonizing the turtle without provocation.
Sokka from Avatar: The Last Airbender temporarily went through this in "Bato Of The Water Tribe" when he yells at Aang for hiding his father's map. Though this is partly justified; Aang DID act dishonestly in hiding the map from him and Katara. Still...that was harsh.
Katara also went through this in a few episodes of Book 3, where she nastily taunts a recently-turned good guy Zuko about losing his Firebending. It's revealed later that she's still angry at him for betraying her at the end of Book 2...that, and she's still upset at the loss of her mom. At the end of The Southern Raiders, she finally lets go of her anger and forgives Zuko.
The Earth King in The Promise Trilogy. Instead of listening to Aang telling him to at least talk to Zuko about why he quit the Harmony Restoration Movement, he declares war instead.
There's also Aang in The Desert, where he lashes out at everyone at the slightest provocation. This is justified due to him losing Appa, who was his closest friend and the last link he had to his dead people.
Lin from Sequel SeriesThe Legend of Korra was never the nicest person throughout the past two seasons, but in "The Metal Clan", she really acts ugly towards her half-sister, Suyin. It's also reached a boiling point when she makes her niece Opal cry by yelling at her to leave her alone. Korra snaps at her for this, saying that she'll always be a lonely old woman who would never change. But then she Took a Level in Kindness in the next episode, makes up with Suyin, and apologizes to Opal.
Kappa Mikey has the titular character doing this in one episode, The Good, the Bad, and the Mikey, when he ended up discovering that USA hero, Captain Impressive, got canned for being a good two-shoes. He eventually realizes the error of his way when he ended up causing a riot.
She and her fellow girls become this in "Equal Fights", going all jerky on the boys after an encounter from Femme Fatale. Cue What the Hell, Hero? moment from Ms. Keane and Ms. Bellum should get them out of this.
Bender of Futurama fame has always been a bit of a jerk, and that doesn't even take into account the fact that he's a criminal for numerous cases of thievery alone. Still, he isn't above a Pet the Dog moments and can feel guilt for some of his harsher actions, and that doesn't even take into account Fry and Leela really are to him. Later episodes, particularly in the show's Uncancelled era, increased his sociopathic tendencies however, where he goes so far as to commit outright murder and at one point even damns his own son to robot hell, something that disturbs the Robot Devil.
Lightning starting from the Total Drama Revenge of the Island episode "Eat Puke and Be Wary", after he accidentaly gives Cameron the win, he effectively replaces Scott as the Big Bad.
Benson took a huge one during "The Best Burger in the World" when he orders Mordecai and Rigby to do their work from the previous week, which ruins their opportunity of them getting their burgers. The same thing to a few other episodes such as "Temp Check" when he threatens to fire Rigby for not doing his work, "More Smarter" when he threatened them with physical abuse, "Replaced" due to firing Mordecai and Rigby for throwing a party, "Muscle Mentor" for not being concerned about Rigby, and "Paint Job" when he docks Mordecai and Rigby's pay for 6 months.
Rigby in "Do Me a Solid" and especially "Wall Buddy".
Mordecai in "It's Time" when he accidentally kills Rigby, although some would say it was Rigby's own fault for being a jerk the whole episode. Also in Laundry Woes when he kicked out Rigby about fighting over returning Margaret's sweater.
Iron Man in Ultimate Spider-Man, was initially a mentor to Spider-Man. However in "Swarm", he seemed much more disrespectful and distant towards Spidey.
Norbert of The Angry Beavers, who became steadily more of a self-centered asshole as the show continued its run.
Princess Bubblegum in Adventure Time was given a few morally ambiguous traits in some of the earlier episode. She sometimes let her anger get the better of her and would mercilessly pursue those who had wronged her. Some of her science experiments also went wrong, such as producing her extremely screwed up offspring Lemongrab. However, more often than not Bubblegum would show regret when she screwed things up, and work to fix her mistakes. Over the course of season 5, her actions turn from well meaning mistakes into outright atrocities, such as committing genocide on a group of defective robots, sacrificing one of her subjects because she thought he wasn't as valuable as Finn or Jake, throwing Tree Trunk's entire wedding into prison just because Tree Trunk's prefered the fake King of Ooo preside over her wedding than her, causing almost the entire conflict of one episode just because she doesn't like people thinking of magic as anything than another kind of science, and in general showing no remorse for hurting people if she believes it's for the greater good. A canon video game also reveals that she trapped and performed painful experiments on the two Lemongrab brothers, when she had previously been shown trying to help them. If she was the main character, she'd be in Villain Protagonist territory by now.