Though he gradually gets better over the course of the season, and is back to his old self by the end powers included.
Guts from Berserk. After the horrible events of the Eclipse that all but broke him, he spends two years as a Byronic Hero who keeps everyone distant from him and is focused solely on vengeance and blood.
In Yu-Gi-Oh! GX, this happens to Kaiser in a big way when he becomes Hell Kaiser, as you can tell from the name change.
Amy Rose of Sonic X was initially a kindly bubbly girl with occasional Tsunderemood swings. By the final episodes she more consistently acts as an obnoxious and occasionally outright violent Stalker with a Crush to Sonic, to the point half her allies, and even some foes, are outright terrified of her. The video games' interpretation bordered a little into this trope as well, though it's usually much more toned down.
Oh Lordy, Nakai from Bakuman。. When he's first introduced he was desperate to get a manga serialization despite not succeeding in ten years. Through much perseverance he proves himself to fellow manga artist Aoki and the two do manage to start up a serialization. However, once he gets a taste of success it goes right to his head and he starts acting like an arrogant, womanizing asshole, alienating most of his friends and colleagues, ultimately getting himself canceled and decides to quit manga and go back home. Recently he's returned to the business, but not only is he working for Nanamine, the first unambiguously "evil" rival in the series, but both his appearance and personality have grown even more slovenly and repulsive during the interim.
Most of the former NERV officers after the Time Skip in the third Rebuild of Evangelion, especially Ritsuko. Somewhat justified though since the reason everyone is now so hostile towards Shinji is because he almost destroyed the world. It's a lot less justified when you take into account that he didn't know his actions would destroy the world, though.
Chiba and Maho from Wandering Son have taken this in recent chapters. In Maho's case, it might be because she hit puberty.
To specify, Saori went through a sort of Trauma Conga Line in late elementary. She was becoming increasingly jealous of Takatsuki's friendship with Nitori which lead her relationship with everyone to sour. Eventually she became bitter and aggressive. Maho on the other hand was under stress of having her crush fall in love with her little brother, along with said sibling being transgender.
Naruto: Sakura in the fillers. While she started to genuinely warm up towards Naruto, the fillers seem to ignore the poor girl's Character Development for the sake of humor. This got to the point where she started taunting Naruto for his misery in the third movie.
When Tobi told Sasuke the Awful Truth, Sasuke went from sparing the lives of insane prisoners to casually murdering guards doing their jobs, not to mention almost killing Karin because she was captured by the freaking Hokage Candidate.
Tobi, the Masked Man, who is revealed to be Uchiha Obito.
Iruka of all people takes one in The Sixth movie. Set after a battle with Akatsuki, some of the Konona 11's parents are offering to recommend to Tsunade that they get promoted to Jonins for their efforts. Naruto seeing this goes to him as he's the closest thing Naruto has to a family and asks if he could do the same. Iruka coldly shoots him down and tells him he won't get special treatment just cause he saved the Village from Pein not even offering to let him get promoted to Chunin or give him a reason why he can't do that. Geez Iruka and I thought you loved Naruto like a brother
Whenever Goten and Trunks fuse into Gotenks, their worst traits combine to make a Smug Super child whose immature showboating is in stark contrast to the individual children, who at least realize when it's time to get serious.
A Justified Trope in the case of Tem Ray in Mobile Suit Gundam. He was already a borderline Mad Scientist in the beginning, but the biggest leap into jerkassery happened due to brain damage from oxygen deprivation, which can really screw up your personality.
Arguably, Medaka of Medaka Box. Since Ajimu has introduced five "normal" people to her, Medaka no longer views Zenkichi as someone special to her since he couldn't pass the first trial of a test she gave the normals. Then she actively tries to turn him into her enemy by acting cold and offensive just so she can see value in him again. Kind of sad considering it's a combination of her abnormality and her personality that causes her change like this since she can't understand failure or things from a "weak" perspective. She even goes as far as using an abnormality that makes her weaker than Zenkichi and goes all out in a fight against him. Since Zenkichi isn't really willing to fight her at this point she mops the floor with him. However the abnormality she used is a "perfected version" of Kumagawa's Bookmaker and isn't meant for fighting, but for allowing the person affected to feel the world from the opponents perspective.
Takanashi from Working is suddenly a lot gruffer in the second season. Justified in that dealing with Inami will do that to you. Then inverted in that he becomes super-nice to her with even less explanation.
Chiaki Yoshino from Sekai-ichi Hatsukoi went from bratty but well meaningMan Child to immature jerkass. This might be justified as he's dating his best friend Hatori who is a Yandere and given that he was brutally raped and then suddenly forced the two into a relationship based on co-dependency pretty much made him worse as shown when his relationship with his best friend Yanase is strained given that he fails to understand Yanase's feelings and not only turns him down multiple times in the worse way possible but constantly rubs salt on the wound. It gets to the point that Yanase does call him out on his attitude...unfortunately, it means nothing in the next episode where Chiaki pulls a huge jerkass moment that the fanbase won't let him live down and generally through him into The Scrappy territory.
The girls of Hinata House, most notably Kitsune and except Mutsumi, during the story set in the Kingdom of Molmol in Love Hina.
Gundam Age, Flit Asuno goes from an idealistic shonen hero to a ruthless dictator who is willing to destroy his enemy "race" without listening to anyone even his son and grandson. To say that he has taken a level in jerkass is an understatement.
Linebarrels of Iron: The main character so incredibly straight until even the helpful support character hates him.
While Yukiteru in Future Diary did eventually grow a spine and Took a Level in Badass, he also becomes an unknowing asshole at the same time due to being tricked into thinking he could bring his parents — and any innocent civilians he killed during the Survival Game — back to life when he became God. Once the truth comes out, it takes five characters to call Yukki out before it finally gets through.