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.
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.
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.
How I Met Your Mother: In Season 6, Zoey Morrison gets Ted's entire class to do this when he tells of his wanting to tear down the Arcadian.
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.
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.
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.
Vanessa and Jenny in Gossip Girl. Jenny gains points for having gone 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 fiancÚ 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.
Phoebe of Charmed seemed to take a level in jerkass at the end of her relationship with Cole.
Admiral Adama drifts this way toward the end of the fourth season of Battlestar Galactica; after the disappointment of the scorched Earth arc, he temporarily regresses from a respected leader into an angry, pathetic drunk.
Actually, BSG seems to have tons of examples. Lt. Felix Gaeta, anyone?
Detective Quinn in season six of Dexter. He had already stolen from a crime scene earlier in the show, but he got worse. After screwing up at work he shows up to his ex-girlfriend/new boss Debra's housewarming party drunk off his ass with a girl he picked up from a bar, and proceeds to act like a douche to all present. It's quite telling that even the girl leaves because he took her to his ex's party simply as a cheap insult, and his partner (Batista) straight-up slugs him when he takes it too far by harassing his sister. In the following episodes he goes out drinking on week nights, behaves like an obnoxious asshole at strip clubs, doesn't show up for work, gets his partner almost killed by getting drunk and sleeping in instead of going to work. 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)
Mike Stivic of All in the Family apparently turned into a jerkass after he, Gloria and Jr. moved to California. We never see exactly how it happened, but he started to ignore his once beloved wife and son, slept around with another woman, compared marriage to Hell and he called Gloria fat.
Well, to be fair he was already pretty much a jerk. And there were a few episodes dealing with him being a tad embarrassed of Gloria's lack of a higher education and in one episode Gloria worries that the above is exactly what would happen one day.
Kellerman in the later seasons of Homicide: Life on the Street, for (arguably) understandable reasons in response to certain plot developments.
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, and gains the sexist attitude that Zack recently got rid of.
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.
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.
Speaking of 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.
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.
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.
Finn Hudson in Glee recently cranked 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 slushy, causing Blaine to need surgery.
Becky was a sweet kid in Season one but under Sue Sylvester's influence became ean and cruel
Artie has shown signs of this in Season 3, especially when he's playing director.
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.
90210: Adrianna in season 3. She even went as far as swapping out Silver's medication for Bipolar Disorder with placebos.
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.
Jack Hunter on Boy Meets World in season 7. He started out basically a Nice Guy who was a little naive due to being new in town and occasionally exasperated with Eric's Cloud Cuckoo Landerness. In the season 5 premiere episode "Brothers," we are introduced to him as someone raised by an upper-class stepfather. His estranged brother Shawn assumes he's a rich brat because of this. However, in the following episode "Boy Meets Real World," while he and Shawn are arguing, Jack admits to working summer jobs in order to obtain the money he has, thus giving Shawn and the viewers a positive impression of him. But he seemed to transform into a Jerk Ass after his breakup with Rachel in the season 7 premiere episode "Show Me the Love." Throughout season 7, he degrades into a narcissistic, spoiled pretty-boy. This is especially lampshaded in the ante-penultimate episode "Angela's Ashes," when Jack is cut off from his father's funding. Rachel tells him he can't go to fancy restaurants, shop at name-brand stores, get expensive haircuts, and such. Jack whines throughout the episode about this, even as he's saying goodbye to Angela who's leaving for Europe for a year with her Dad.
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.
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. On top of that, she was often portrayed as being in the right and getting away with being an asshole to everyone. 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.
Pierce and Abed respectively went through this in the second and third seasons of Community. Pierce started out as a fairly harmless Cloud Cuckoo Lander who eventually, after feeling his friends will leave him eventually, turns into a total Jerk Ass 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.
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.
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.
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.