Jack Bauer from 24 was originally a soft-spoken character who rarely raised his voice, at least for the first season. But once season 2 comes along, Bauer turns into a complete jackass following the death of his pregnant wife and after he was tortured by terrorists, and by the end of the 3rd season, he has turned into a complete douchebag. He does make a conscious effort to undo this in the later seasons - only for him to go through even more trauma which causes him to take at least another 20 levels. Not that you can blame him, though, what with the high amount of loved ones he's lost, agonizing tortures he's gone through, and so-called "friends" who've repeatedly betrayed him..
90210: Adrianna in season 3. She even went as far as swapping out Silver's medication for Bipolar Disorder with placebos.
Happens with princess Xenia in the Czech tv-series Arabela. Once her father leaves for the human world, she goes from a somewhat mischievous and vain girl to a complete tyrant.
Admiral Adama drifts this way toward the end of the fourth season of; after the disappointment of the scorched Earth arc, he temporarily regresses from a respected leader into an angry, pathetic drunk.
Tom Zarek. Early in the series, he was a recurring political provocateur and alleged terrorist, but he actually managed to be somewhat nice towards the main characters, especially in the Kobol and New Caprica arcs. Flash forward to mid-season 4, and he's perpetrating a mutiny among the fleet and having the Quorum of Twelve gunned down in a petty power-grab while President Roslin is in the midst of a Heroic BSoD. Fortunately, he doesn't survive it.
The Melchetts of Blackadder II and ... Goes Forth greatly differ personality-wise: Elizabethan Lord Melchett is (with Lord Blackadder) the Only Sane Man (aside from a familiarity with a certain sheep). The WWI Melchett however, is a General Failure.
Blackadder himself underwent this between the second and third series. While the season 2 Lord Blackadder was certainly ruthless, dishonest, abrasive and grasping, he was also in the court of The Caligula and was in clear fear of his life in multiple episodes just from his own sovereign's insane whims. Edmund Blackadder, Esq. in season 3 has a troublesome but reasonably secure position as the Prince Regent's right-hand man, and uses his limited authority to constantly torment his underling, get away with multiple murders, cheat his employer, and even kicks a cat at one point.
Priya and Raj in season four. For Priya, she basically forbade Leonard and Penny to hang out together due to jealousy, then she lies to Leonard about not moving back to India. For no adequate reason. Karma Houdini doesn't cover it. For Raj, he begun to have lust for Howard's fiance Bernadette (and when being told a rumor of them breaking up, wanted to date her even though Howard possibly wouldn't let that fly) and then after Penny falling asleep before they could have drunken sex still acted like an ass that slept with her.
Jack Hodgins for part of Bones season 11. He was paralyzed in an explosion and lashed out at everyone including his wife Angela. He didn’t think anyone understood what he was going through and that Angela deserved better than someone who was paralyzed. He finally asked for a divorce and she finally got through to him and he improved his attitude somewhat.
The longer the series goes, Walter White gains new and new levels of jerkassery, but after killing Gus, there's barely any humanity left in Walt except when it comes to his family.
Not overtly in personality but he's much shadier in his lawyer tactics in Seasons 2 and 3 compared to 1. He mostly cleaned up his Slippin' Jimmy attitude at the start of the series and only relied on cons because he was desperate. After reuniting with his old friend Marco at the end of Season 1, he starts conning because he likes it. Jimmy becomes noticeably colder and more cynical in Expenses. First in the scene in the bar where Kim becomes unnerved at his proposed con with the rude customer, followed by his dismissal of her concerns about what they did to Chuck. Then, when he meets the insurance agent, he uses Crocodile Tears while letting slip that Chuck was incapacitated so that the agent will jack up Chuck's premiums and maybe have him investigated. He's also more aggressive and opportunistic in his usage of Slippin' Jimmy tactics such as screwing over the obnoxious music store owners, the community service supervisor, and Irene Landry. As his probation and disbarment stretch on, however, he becomes more and more like Saul Goodman, and eventually starts using the name for his street dealings, proudly selling untraceable phones to known drug dealers and unloading his residual guilt about Chuck's death onto Howard, leaving the latter a broken wreck. Taken even further in Season 5. He is invited to lunch by Howard, who in good faith expresses remorse for siding with Chuck all these years, attempts to bury the hatchet with Jimmy, and offers him a job at HHM. Jimmy's response is to take offense at the idea of Howard pitying him and secretly smash up Howard's car with a bowling ball later that same night. It gets even worse in season 6 Gene goes deep into self-destructive awfulness in the episode "Breaking Bad" after a painful phone call with Kim, remaking himself as Viktor, showing no shame for scamming a cancer victim and dismissing concerns from Jeff and Buddy. Even Saul, famous Stopped Caring mask himself, shows concern for Jesse being treated badly in the same episode.
For much of Season 4 Hank takes out the pain of his violent trauma and humiliating paralysis on Marie, showing no appreciation for anything she does to help him while he's bedridden, coldly rebuffing any attempt she makes to reach out to him and generally treating her in an emotionally abusive manner. After Walt drunkenly gives him a new lead on the Heisenberg case, Hank reading his notes at the table and politely apologizing for making a mess is a clear sign that He's Back!. This trope hits him hard when he finds out Walt is Heisenberg and he completely loses his sense of humor or any sign of bantering with his colleagues including Gomez, the only exception is when he's arresting Heisenberg.
While Marie was already on the road to this by the end of Breaking Bad, her fury over Walter dying before he could receive his legal comeuppance and grief over Hank's death has made her a lot more curt and unpleasant. This not only extends towards Saul (for whom her unfriendliness is very justified), but towards the prosecution when she feels they aren't moving the direction she wants them to.
Mike Ehrmantraut, as depicted in Breaking Bad, is mostly a pitiless, thuggish monster despite some standards and the love he has for his family. For the entirety of Better Call Saul, he is much more of a conflicted, empathetic person despite being a hardened criminal willing to be ruthless when forced into a corner. Compare his gentle approach to calming a traumatized Jimmy, who was just in the middle of a gunfight he almost got killed in, to him slapping a despondent Jesse, who just witnessed Jane's corpse, into getting his attention. Although many fans long chalked this up to a case of Characterization Marches On, his last scene in the latter series suggests he took Manuel'swords to heart and embraced his lot in life as a mere gangster with no genuinely meaningful ethics. Lending credibility to this, the first time we see him in the Breaking Bad timeline in Better Call Saul, his personality is perfectly in line with his characterization in the former series, being very ill-tempered and threatening to cave Saul's skull in for joking with him.
Bolsa became a much worse person to work under. Notably, his professionalism and tolerance for bullshit degrades heavily, and he is much more spiteful and angry towards betrayal in any form.
Domingo's naivety and nervousness gradually disappear as he becomes more accustomed to life as a Cartel dealer and informant. Compare his meek behavior when in front of Lalo Salamanca while playing cards with his manipulative, vengeful behavior when locked up in Walter White's basement.
Once Charles McGill's true colors have been revealed, he seems to be openly trying to sabotage Jimmy's new job at Davis and Main. He even shows up to joint meetings between them and HHM, seemingly just to mess with Jimmy. He is also really haughty toward Ernesto and Mesa Verde.
As Kim becomes increasingly comfortable with engaging in shady activities with Jimmy, she becomes spiteful towards Howard Hamlin to the point of wanting to ruin his reputation and possibly a career.
Brittas was characterized from Series 2-5 as a Pointy-Haired Boss who was nonetheless given moments where he clearly cared about his staff and the customers, and who was motivated by the desire to make the world a better place. Series 6 gives him much fewer of these moments and he becomes notably more apathetic towards customers who have been seemingly crippled or traumatized for life as a result of using the leisure centre. The fact that Laura had been Put on a Bus at this point didn't help matters.
Whilst Tim was already characterized as a Crazy Jealous Guy, at no point in the previous 5 series does he go as far as pushing Gavin into a pool just because he apparently might have a crush on Penny. Nor does he go as far as holding the other staff members hostage because he found out that he gets paid less than them.
Whilst Linda was shown to have a dark streak involving guns, she was otherwise characterized as a loyal member of Brittas' team and one of the kinder members. Come Series 6 and 7 and she becomes a Soapbox Sadie willing to commit acts such as locking Tim up in a cage, stealing and setting free a shark, and setting free Colin's pet rabbit under his nose to prove a point about keeping animals in captivity.
Ben, Carole's son, goes through this the worst. From Series 1-5, he was characterized as a normal toddler prone to the occasional bratty and Troubling Unchildlike Behaviour, but otherwise harmless, even saving Brittas from the collapsing leisure centre at the end of Series 4. In Series 6, he's a downright Enfant Terrible, with his scenes involving him either arguing with his mother or outright committing deeds such as holding several girls hostage for party bags, maiming a party entertainer possibly for life with a lawnmower, biting other children, and dangling a teacher out of a window.
Buffy had become increasingly arrogant over the last two seasons. It's partially justified in that being yanked out of Heaven and coming to Buried Alive in her own grave had been very traumatic. However, by the last season she had practically declared herself dictator of the group and made sneering comments when one of the girls under her charge committed suicide. This is hardly the first time she does. First she is stressing over The Master, and her Jerkassness as he starts to affect her would impress Faith. Later everyone took a level after she runs at the end of the second season. Justified in the fourth season as a demon was stealing her soul. And in season five a magical retcon gives her a sister, and Buffy struggles with this. Buffy's behavior in S7 can't even be justified by the Back from the Dead trauma, as the end of S6 showed her overcoming it. She even goes so far as mocking a teenage girl for being Driven to Suicide as well as accusing Willow of being "useless" for being hesitant to use magic. Keep in mind that not only did Willow, less than a year beforehand, go Omnicidal Maniac due to magic use (which therefore kinda justified her hesitation), but she also tried to use magic against the season's Big Bad, and was subsequently possessed for her troubles. A possession that Buffy saw with her own eyes and that convinced her to tell Willow that they wouldn't use magic until they were positive it would be useful. Little cruel to mock her for it later, Buff.
Faith, she does this to a frightening degree after she accidentally kills someone, to the point where Buffy is happy to kill her. She gets better following her first Angel appearance, however, to the point that she's one of the nicer characters by the end of the television series.
Speaking of Angel, Wesley inverted it before taking a higher level that went side by side when he increased his badass levels. On Buffy he was a pompous, smug jerk that everybody found to be a pain in the ass, but after he was fired from the Watcher's council and moved over to Angel he'd taken a slice of humble pie and became far more sympathetic. Then season three came and put him through the emotional wringer. After that, the same generally nice guy from the first two seasons ruthlessly stabs a harmless junkie at one point in order to make her reveal what she knows about Angelus. And tying in with Faith cooling down from her own Jerkiness levels, she actually winds up being horrified at his actions.
Amy has this happen throughout the series with her treatment of the main characters, for no apparent reason. She starts Season 1 out as a *victim*, and the Scoobies save her. In Season 2, she is a witch herself whose worst offense is trying to cheat on her homework. In Season 3, she is into magic, and is still friends with Willow. An actual Witch-hunt causes her to turn herself into a rat to escape. She stays that way, more or less, through Seasons 4 and 5. In Season 6, after being turned back into a human, she escalates Willow's magic addiction. In Season 7, she actually curses Willow in a manner that will likely cause great suffering, all because "magic came so easily to Willow, when the rest of us had to work hard at it". This, despite the fact that Willow spent several seasons complaining that she could do little more than levitate a pencil, whereas Amy was able to spontaneously enthrall people and turn them into rats.
Georg Tischendorf in Charite develops for the worse after entering a fraternity, getting increasingly more condescending and arrogant, developing his nationalism to nasty extremes and revealing that, contrary to what he always told his sweetheart Ida, he doesn't care at all for her plans to become a doctor and thought those were "a female whim"; in his opinion, she should become his good, docile wife.
Phoebe of Charmed seemed to take a level in jerkass at the end of her relationship with Cole.
In Season 5 of Chuck, Morgan, the title character's friend, turned into quite a self-entitled asshole, brushing off Chuck and the rest of his team, and even going as far as joining a rival spy firm. However, Morgan himself isn't at fault for it, as the Intersect that he gained at the end of last season was "tainted", causing him to lose himself, so to speak. He eventually had it removed after four episodes or so.
Pierce and Abed respectively went through this in the second and third seasons of Community. Pierce started out as a fairly harmless Cloudcuckoolander who eventually, after feeling his friends will leave him eventually, turns into a total Jerkass who goes so far as to try and egg on a suicidal student. Abed wound up embracing his dark side over the course of the third season which saw him regularly insult his friends and often do whatever he wanted regardless of any of their feelings.
Control Z: Pablo when he is seeking revenge against Raúl and Susana after becoming principal at National School.
Tracy Barlow is possibly the show's Trope Codifier. Initially, she was the precocious and likable daughter of Deidre and Ken, even though she eventually became a rebellious teenager who took ecstasy which caused kidney failure and married young. That being said, she still was nothing compared to the cruel, man-eating bitch she became after her return to the Street in 2002 and some of the worst things she's done include trying to sell her unborn daughter, coming in between her childhood sweetheart, Steve, and both of his wives and killing an ex-boyfriend due to his "abuse" of her. And now, we can add serial killer to her list, which she framed her rival, Carla, for whom she almost killed along with her own flesh and blood, Amy. The ultimate Jerkass indeed.
Nowadays, we have Sarah Platt and Todd Grimshaw. Although originally sympathetic teen mother Sarah was showing signs of this trope before her departure by turning into a snooty slut who cared more about her latest hook-up instead of her child's welfare, Todd's behavior, which went from the sensitive, straight-A gay studentnote In fact, he was the first gay character in the soap's history to a troublemaker, almost came out of nowhere. In between the both of them, she has raised a smart-ass, trampy daughter (who looks much older than a fourteen-year-old) and he has stolen money from his mother and broke up a relationship of a "friend" of his just so he can be with her boyfriend. It is almost that they are not the same people from before, just like when Ryan Connor came back as a much less likable (and attractive) character versus the studious, kind boy he was before. Also, it's rather telling that since their return, neither of them has been to their son's grave...
Criminal Minds: Season 1 serial killer Karl Arnold is shown again in season 5 episode "Outfoxed". Sure he wasn't a nice guy when they caught him, but four years later, he's got even worse.
We've seen this happen a few times on Deadliest Catch as various crewmembers' clout increase, especially in season 6:
The Northwestern's resident Woobie/Determinator, Jake Anderson, wanted to go to the captain's chair after his climb up the crew ranks at the beginning of the season, prompting Captain Sig Hansen to swap him for a few weeks with Jake Harris of the Cornelia Marie, who was reaching a boiling point with his elder brother Josh.
However, in season 7, when Jake is being taught how to be a captain by Sig, even allowed to run his own string of crap pots, he shows surprising humility. When his string comes up almost blank, he blames himself more than anyone on the crew and resolves to work harder to make up for it. A distinct example of losing a level in Jerkass.
Inverted on the Time Bandit, where the other crew started treating Mike Fourtner badly when he was actually given a test run in the Captain's chair.
Day Break (2006): When Hopper first clears his name, the day resets anyway. He becomes so despondent that he finds Safety in Indifference, taking Rita to Las Vegas and then Mexico for two weeks straight until he gets so bored with that he tells her the truth and that nobody else matters, laughing off that she will even remember the conversation they just had. When the day *does* change to reflect Hopper's bad behavior, he goes back to being his old heroic self.
Desperate Housewives: Julie Mayer suddenly becomes hostile and resentful toward her mother after four seasons showing a close, best friends-type relationship. The show's sweetheart drops out of medical school and sleeps with a married man, then gets pregnant by a guy she used to babysit.
Detective Quinn in season six of Dexter. He had already stolen from a crime scene earlier in the show, but he got worse. After he screws up at work he shows up to his ex-girlfriend/new boss Debra's housewarming party stone drunk along with a girl he picked up from a bar, and proceeds to act like a douche to all present. Even the girl leaves when she finds out he took her to his ex's party as a cheap insult, and his partner Batista straight-up slugs him when Quinn takes it too far by harassing Batista's sister. In the following episodes he goes out drinking on week nights, proposes to strippers, doesn't show up for work, and eventually almost gets Batista killed by getting drunk again and sleeping in. When he's fired for it, he manages to stay by exploiting a loophole intended for helping alcoholics. (He is an alcoholic, but only because he's such a Jerkass, not because he actually needs help) He gets better in seasons 7 and 8 though, so his season 6 behavior seems to be mostly getting over the break-up with Debra.
Underplayed in the Third to Fourth Doctor regeneration. The Third Doctor was stern and occasionally witheringly cruel, but very fatherly and caring, and one story clarifies that he views Earth as his home and himself as an "immigrant". When he regenerates into the Fourth Doctor, he becomes a dorky, jolly, kooky figure... with Blue-and-Orange Morality and no particular identification with humanity or Earth. Sarah Jane is shown to struggle — despite her best efforts — to get on with him at first. Eventually he and Sarah become very close, but he'll still get scenes where he fails to understand why he's expected to mourn that a human was just murdered, or that Sarah might be happy to have someone with her when she's scared.
Even with the above, the Fourth Doctor is also shown to be quite introspective and gentle, getting occasional scenes where he'll show deep caring for Sarah or emotional generosity towards a guest character. He talks about how much he loves humans, and at one point Sutekh is even shocked by the extent of his affection. However, when Sarah leaves, the loss of her Morality Pet function and a move towards the Bloodier and Gorier brings out his less admirable traits, making him much more violent as well as prone to bullying girls, and then a sudden Lighter and Softer backlash removed the high stakes and gritty tone that made this forgiveable. By mid-Season 15, he's a smug, loud, temperamental, arrogant comedy Narcissist who treats everyone around him like crap, even characters he's supposed to love like Leela and K-9. He is toned back down in Season 17, which gives him the opportunity to be obviously affectionate and equitable towards Romana and various guest characters.
After the Fifth Doctor, the kindest and gentlest incarnation so far, got broken, he regenerated into the Sixth Doctor, a crazy raging egotist with an exceptionally abrasive and rather violent personality.
For the first part of the episode, the Doctor is noticeably more cold, strange and egotistical than usual, and at one point Jack has to tell him to remember his social niceties. This could very well be because the Doctor is adjusting to Jack's presence and nature as a Fact of the timeline, something which does the Bizarre Alien Senses equivalent of making his hair stand on end.
Professor Yana accomplishes this in less than a minute when he opens his Chameleon Arch and is turned back into The Master. Within seconds of transforming, he's sabotaging his ego's life's work and murdering said ego's best friend without a shred of remorse.
"Turn Left" makes it crystal-clear that Donna Noble mellowing out as a person was due to the Doctor's influence. In an alternate timeline where she never met him, she's just as stubborn and obnoxious as she was in her first appearance, "The Runaway Bride".
"The Time of Angels"/"Flesh and Stone": While the Weeping Angels have always been described as psychopaths, in their first appearance the Doctor described them as the "nicest" psychopaths in the universe because their method of killing was simply to send their victims into the past with a touch. The Angels here engage in Evil Gloating and kill people by breaking their necks since they currently have a different source of energy and don't need the temporal energy that could have been gained from their victims.
Much of the arc of Series 8 is about Clara dealing with the fact that the cuddly, quirky young man she'd previously been travelling with is now a barking-mad, fifty-something Scotsman with No Social Skills, a stormy personality and a total lack of understanding of and interest in humans.
On Everybody Loves Raymond, Debra started out in the early seasons as a fairly reasonable everywoman who seemed to have a quite playful and loving relationship with Ray. Then, the character suddenly changed. It's hard to pinpoint exactly when the change occurred, but Debra eventually became a raging, physically-abusive and smugKarma Houdini who treated Ray like trash, beat him up, forbade him from seeing his friends most of the time, and convinced Ray's own children to feel contempt for him. Oh, and the worst part? The show tried to act like she was the reasonable one, despite her abusive, Jerkass behavior. So basically, she went from being a well-rounded, likable character in the beginning to the writers' Mary Sue by the end.
Phoebe, way more so. She becomes increasingly more shrill and mean-spirited as the seasons went on.
Game of Thrones: When recruiting Northern Houses to join the cause in Season 6, Sansa expects them to obey despite having nothing but an oath to a House that no longer exists in a fight that seems pointless, though it is pointed out that if the Boltons win, the North is screwed when the White Walkers arrive. She also criticizes Jon for not having more men despite having gotten all they could gather before snow sets in, not telling him about the Vale army that's waiting in Moat Cailin for her word. She apologises for this later.
Finn Hudson in Gleecranked it up a notch. After whining about being cheated on by his last two girlfriends, he eagerly began an affair with one of them while she was still in a relationship with Sam Evans.
Sebastian Smythe is another example. Sure, he was always a jerk, but mostly in the "lovable rogue" sort of way. Some people even shipped him with Blaine or Kurt or both. Fewer people do now thanks to him tampering the slushie, causing Blaine to need surgery.
Becky was a sweet kid in Season one but under Sue Sylvester's influence became mean and cruel.
Artie has shown signs of this in Season 3, especially when he's playing director.
Goodbye My Princess: Cheng Yin starts out a genuinely nice guy. But when he decides he wants to be Crown Prince, he becomes more and more cruel.
Jenny in Gossip Girl goes from sweetheart-everyone-adores to being hated by pretty much all of the NJBC.
Blair Waldorf was never a nice person per se but even many of her devoted fans had enough of her by season five. She treats her fiancé horribly yet somehow feels he is the bad guy for not putting up with all of her whims (including briefly running off with her ex-boyfriend Chuck and professing her eternal love for Chuck minutes before the wedding). She claims to love Chuck but treats him like dirt under her shoe. She spends several episodes trying to help her best friend Serena win back Dan, then turns around and begins to date Dan herself (while still married), making out with him in Chuck's bedroom and right in front of Serena moments after Serena's grandmother died, even though she doesn't have romantic feelings for Dan. Then there's the fact that she treats Dan with little to no respect while they're supposedly friends or while they're dating. And let's not forget that she doesn't care that her own baby died but she's terribly depressed that when her two month old marriage to the prince ends she no longer gets to be a princess.
Dan Humphrey is the prime example from Gossip Girl. He started out as the show's moral compass who always believed in doing the right thing. Flash forward to season three where he's dating his lifelong best friend Vanessa but cheats on her with Serena (who, at the time, was dating Dan's only male friend Nate) and doesn't seem to think he's done anything wrong. When Vanessa for some reason wants to get back together in season four he begins to sleep with her and lets her think they're back together, leaving her to take care of the son he has by Georgina Sparks while he runs after Serena. By season five he has fallen in love with Blair and could care less that she drove his sister Jenny out of town. He then publicly humiliates Blair at her wedding by sending in a video to Gossip Girl which he blames first Chuck and then Serena for. He then tells a newlywed Blair that unless she begins to date him (Dan) he will no longer be her friend. They do date for a while, but then he cheats on her with Serena. When he finds out Blair is leaving him for Chuck, he makes good on his promise to not be her friend unless he can be her boyfriend and sets out to write a book that will uncover all her secrets (and the secrets of the rest of the NJBC) because he feels wronged. So much for moral compass...
Dr. Eric Foreman on House, who started off as a fairly self-possessed guy who didn't really appreciate House's shenanigans, and then progressed to the type of guy who screwed over a colleague and then stabbed her with a dirty needle (the latter under duress, but still). His jerkassery has calmed somewhat, but hasn't completely gone away. To be fair, the growing jerkassery did become a plotpoint after that — Foreman was afraid that spending so much time around House was causing him to become more like him. He quit Princeton-Plainsboro to try to stop this... but then got fired from a job in New York after using very House-like tactics. Cuddy even once called him "House-Lite."
House's own jerkassery grew out of control as the show went on.
Patricia, in the first few minutes of the season 1 premier, was shown as a normal, relatively nice girl. As soon as Joy disappeared and Nina showed up, however, she turned into a bully for a few episodes.
Nina in season 1 was a perfectly nice, forgiving and adventurous girl. In season 2, due to stress and fear, she grew a temper and would continuously lash out at her friends and Fabian. She also became more prone to jealousy, getting angry when Fabian so much as talked to Joy.
Joy as well, having progressed from a dramatic-but-normal girl to someone so jealous and selfish, she was willing to write a toxic article on Nina and publish it under the name Jack Jackal, Mara's writing alias. This hurt Nina, and put Mara in danger from being taken off the blog again.
Jerome, after spending the entirety of season one and two learning to be a better person, went back to a minor Jerkass nature when he cheated on Mara with Willow. Mara herself took a level when she let her pain and desire for revenge consume her, leading her to continuously humiliate and hurt Jerome over what happened, and to bully Joy as well for developing feelings for him (as opposed to simply faking her feelings for him, as was meant to happen for their final revenge plan.)
As Sinners are The Soulless, this is pretty much what happens: they lose their conscience and thus they pretty much become sociopaths.
How I Met Your Mother: In Season 6, Zoey Morrison gets Ted's entire class to do this when she tells them he's designing the building that will replace the Arcadian. She incites them — though they liked Ted very much up to this point — to boycott his class to protest the planned demolition, which is absolutely NOT his doing as he's only designing the replacement building and has nothing to do with where it's being built. In the end, he reaches his limit and flatly tells his students that anyone who doesn't come to his class will get an "F".
Barney does one in season 5 after breaking up with Robin (which is saying something, as he already was a pretty big jerkass, albeit one with a Hidden Heart of Gold). He goes back down to his pre-Robin level of jerkery in season 6 though.
iCarly: Sam was a bully from episode 1, but she takes level after level. A couple of her points include the episode she bashed Freddie with a tennis racquet and chucked him out of a treehouse for a differing opinion, and then later starting a basement sweatshop using elementary school kids to mass produce their Penny Tee Shirts.
Continued in Sam&Cat. While Sam was shown to be mostly aggressive in iCarly, she became "psychologically" mean and sometimes downright cruel in the later show. Reminded this troper more of Jade West from Victorious than of old Sam from iCarly...
Freddie takes a level in "iCan't Take It", while he is dating Sam, he edits Carly out of a webshow skit (something he never did even to Sam when they hated each other), and taunts her about being jealous of their relationship.
Carly herself especially as the show moved on, though still not nearly to the degree of Sam especially or even Freddie in the example above.
I, Claudius: A number of characters grow surlier as the series goes on or simply drop the more polite facades they'd initially adopted. The biggest example in the series is Tiberius, however. He's introduced as a sour and grumpy man who has a number of sympathetic moments with his brother and wife but after he's forced to divorce his wife and his brother dies, Tiberius spirals into villainy. As the ruler he abuses his power regularly, rapes Lollia, and executes any who speak out against him. At the end of his reign, he takes Caligula under his wing, fully aware of how much a dangerous monster he is and eager to have Rome suffer under him.
Jessica Jones (2015) started with Trish Walker being a ball of positivity like her comics incarnation, an idealistic counterpart to how adopted sister Jessica is a cynical Broken Bird. In season 2, she becomes a crazed addict after continuous use of the combat enhancement inhaler, culminating in her threatening and manipulating those around her all in an effort to gain powers, and ultimately driving them away for becoming so unpleasant. And once she actually gets them, it gets worse, as season 3 has her going on a narcissistic vigilantism career, done more to make her feel beloved than to solve crime, that kills a few people and ultimately lands Trish in jail.
Kengo from Kamen Rider Kiva disappears after a couple of tragic events, then returns several episodes later with special training and a chip on his shoulder, especially regarding Wataru and Nago. Thankfully, he gets better.
As the series progress it's become apparent that the only remotely likable characters in the series are: Lou, Danny, Spencer, and Holly.
Mad Men's Harry Crane started off as one of the nicer guys in the office, compared to his more overtly sexist and meaner co-workers; the worst thing he ever did was an immediately regretted one-night stand that left him sleeping on the couch for a while. Foreseeing the value of television in advertising's future, he talked his way into becoming the head (and sole member) of the television department. The promotion quickly went to his head, and eventually he was cheating on his wife shamelessly and repeatedly, constantly making gross comments, and trying to coerce his way into sex with aspiring actresses, one of them being Don's ex-wife. By the end of the series, every other character seems to loathe him, and they take immense glee in selling the firm days before he would be made partner, cutting him out of a fortune.
Morgana on Merlin. She always was rebellious and butting heads with Uther, but it was because she didn't like his treatment of magic users and was upset with the rumors of him offing the man she thought was her father. But, after she found out Uther was really her father and had lied she went full on evil. Even when Arthur became king (after she had Uther killed), she didn't stop because she views Arthur as being just like Uther and believes only she can bring back magic. Still, she's more evil than anything else now. By the final season she has seeming forgotten about bringing magic back and now only wants to be Queen for the power. To the point where she’s a vicious warlord leading an army of ruthless barbarians, and even starts abusing magic users herself.
Angela Moss of Mr. Robot started off as a Nice Girl. But by the time Season 3 came around, she became an increasingly selfish, sociopathic and cruel Jerkass bully after being brainwashed by Whiterose and became abusive towards Elliot by working with Mr. Robot and Tyrell for Stage 2 and exploiting his mental illness without feeling any regret. Later, when Elliot and Darlene find out about her betrayal, she began playing the victim card and remained unapologetic for her betrayal. After the cyberbombings, when Elliot tried to reconcile with her and snap her out of her fantasy, she outright hated him when she thought that he was manipulating her into telling him Whiterose's plan after seeing Leon in his apartment.
Seth Cohen from The O.C. becomes noticeably more self-absorbed in later episodes. Arguably this helped round out his character a bit more and explain his previously inexplicable unpopularity. In season four, following Marissa’s death, he becomes a lot more tolerable in an effort to support Summer and Ryan.
Ryan was one of the more likable and sympathetic characters in the first few seasons of The Office (US), but his sudden rise to power at Dunder-Mifflin coincided with a massive increase in the size of his ego. His humiliating, highly public fall from grace only seemed to make things worse.
This also seems to have happened with Pam, albeit to a lesser extent.
Andy presents an inversion — originally Jim's new foil (or "Dwight 2") when he transfers to the Stamford office, Andy has become the "all-around nice guy" in Scranton following anger management courses and (more likely) Executive Meddling to avoid having two Dwights. However, it's eventually played straight with him in the final season, where he starts treating Erin quite poorly and becomes rather obnoxious to the rest of the staff. He gets better again in the last few episodes, though.
Alex in Power Rangers Time Force seems like a nice guy prior to being Not Quite Dead. When he appears later in the series, he's turned into a cold jerk who doesn't listen to anyone else. This could be due to his near death experience, possible changes in the timeline, an obsession with protecting said timeline, or any combination of the above.
Roseanne's titular character started out as a gruff, but otherwise friendly and wisecracking blue-collar woman who loved her husband and kids, and slowly transformed into a cruel, selfish, sniping, raging, shrieking bitch with severe misandrist tendencies. Her daughter, Darlene, seemed to follow her into this mold. Given that this dovetailed with the men of the show (particularly Leon, Mark and David) becoming more and more pathetic, useless, and stupid, and that the show was practically written and run by Roseanne herself, it's a pretty clarifying look into her mindset at the time.
Nate from Six Feet Under, even though the personality changes are very gradual and subtle. His idealistic, free-spirited nature doesn't mesh well with his family responsibilities in season 5. But then, given the circumstances...
In Smallville, Chloe takes a few in the later seasons. Jimmy—her doting Nice Guy boyfriend and then husband—had evidence that Davis was a psychotic serial killer, and not only does Chloe refuse to give Jimmy the benefit of the doubt, but it becomes blatantly obvious that she indeed does harbor romantic/sexual feelings for Davis, despite having just married Jimmy. So from Jimmy's perspective, not only does his wife refuse to believe his warnings that this other guy is a murderous psycho, it becomes blatantly obvious that she's been (and continues to be) lusting after this other man. Jimmy was put in a nightmarish situation that broke his heart and left him isolated. His friends all treat him like he's gone crazy (even though it turns out that Jimmy was actually right about everything) and Chloe isn't there for him. His dumping Chloe was completely understandable and warranted.
In Star Trek, Gowron started out as a Jerk with a Heart of Gold, having several Pet the Dog moments to show that he was a much more honorable and reasonable candidate for the position of chancellor of the Klingon Empire than his opponent Duras. After the conclusion of the Klingon civil war and he officially became chancellor, he started to become more and more of a jerkass, such as repeatedly being an Ungrateful Bastard to the Federation, despite owing his position to the Federation's covert assistance. By the time of Deep Space Nine he had become as bad as Duras, even leading the Empire into a war against the Federation simply because he refused to admit he'd been wrong. His undoing came when he was willing to sacrifice his greatest general and the entire Alpha Quadrant rather then face a non-existent threat to his position. Worf decided enough was enough and killed him.
Dave Hester on the reality series Storage Wars was already the resident jerkass, but upon returning to the show after settling his legal dispute with the show's producers, he's cranked it up about three levels, going well out of his way to antagonize just about everybody, including the show's resident auctioneers.
Cody Martin has been known for most of The Suite Life franchise as socially-awkward but lovable nerd. One couldn't help but root for him in the first season of On Deck as he fought with painstaking care to win Bailey's heart. Then, in the second season, he suddenly became full of himself, constantly fought with Bailey for dominance in the relationship, and even assumed Bailey and former love-interest Jessica were jealous of one another, despite a complete lack of evidence thereto.
When he and Bailey broke up and Zack got a girlfriend, after which he starts making fun of Zack for having a girlfriend, he gains the sexist attitude that Zack recently got rid of.
S.W.A.T. (2017): During season 4, as Chris competes with Street and Tan for a prestigious TLInote Tactical Leadership Institute position, she's hostile and aggressive, with Street even raising the issue with her. It seems to be a result of anxiety over fearing that if she loses, it will hurt women's status in SWAT and or the police force overall. Chris mellows in time again.
Nate from Ted Lasso starts off as a timid, well-meaning kit manager who Ted realizes has a good mind for football strategy and later promotes him to assistant coach. In Season 2, it's immediately shown that he's become quite harsh in his new role as a coach; in just the first episode, he initially refuses to let Will, the new kit manager, leave work a bit earlier for his mother's birthday until Ted overrules him, and his first suggestion for handling Dani's "yips" is to show him his paycheck "for motivation" (implying Dani should just get over it). Ultimately, he gets worse over the following episodes and it leads to him making a full Face–Heel Turn when he lets his ego get out of control.
Alan in Two and a Half Men started off as a down on his luck man whose wife threw him out and took him for everything he had, his brother and mother are narcissistic assholes and his son became a moron over time. But in later seasons, Alan became a sleazy weasel who refuses to pay for anything and made his brother pay for everything until he was pushed in front of a train in Paris. And yet he makes himself out to be a Nice Guy and the Only Sane Man.
Dan Moroboshi, protagonist and human form of Ultraseven, went through this when he returned as one of the main characters of Ultraman Leo. While in his own series, he was a compassionate man who cared very much for his teammates and had sworn his life to protect the inhabitants of Earth, in Leo, he's shown as bitter and harsh, especially towards Gen Ohtori/Ultraman Leo who he puts through Training from Hell and berates for doing things like saving children over fighting the monsters. However, at the start of the series, he had been permanently crippled and lost his ability to transform, so the change in character might have something to do with thatnote Not to mention that Kohji Moritsugu's character in Ultraman Leo wasn't even meant to be Dan initially.
Klaus in The Umbrella Academy (2019) went from being the fan favorite in the first season to a narcissist in the second by starting a cult in the 1960s without consideration as to what his followers were leaving behind. Even worse, he engaged in a threesome with questionable consent as they likely wouldn't have had sex if they knew who he really was. Add to that him refusing to tell his siblings that Ben was still with him until Ben possessed Klaus and Diego could talk to Ben; and shortly afterward Ben fully transitioned to the afterlife when he saved Vanya's life. While it was somewhat justifiable because Ben possessing Klaus made the latter violently ill, it had to have been heartbreaking that none of the siblings were able to talk to Ben before his transition.
Jade West of Victorious in the second season. In the fourth episode, she sabotages Tori in an effort to steal a role in a musical by forcing her to repeatedly donate blood Robbie needs for surgery, and in the fifth episode, she attempts to wreck the prom Tori created because Tori unwittingly held it on the same night as the one-man show Jade was planning. Possibly justified by the fact that Beck, who usually calms her evil side, hasn't been around as much this season.
To be fair, since the main cast are portrayed as being either sophomores or juniors in high school, what reason does Tori have for even wanting a prom when it wasn't her time? Not that it excuses Jade being Jade, but Tori more or less ruined Jade;s big night.
Tori herself, and her friends (i.e., almost the entire cast) and arguably, the show itself all took a level in jerkass in relation to Trina as the show went forward.
Similar to the Suite Life example is the story of Justin Russo of Wizards of Waverly Place. Justin was originally just a lovable nerd whose over-achieving sometimes got the better of him, and later on a badass who would fight to the death for his loved ones. Following the "Wizards vs. Werewolves" special, Justin became his sister Alex's bitter arch-rival.
After a series of personal tragedies are piled on top of his already stressful workload, Giardello becomes increasingly irritable and bitter throughout Season 3.
Kellerman starts off as an idealistic, jovial Nice Guy only to become increasingly neurotic and mean-spirited after being forced to deal with false allegations of corruption, which eventually drives him to become a Dirty Cop for real.
Mary Sue by the end.