The Child of Love: The more hurt and more distraught Asuka feels the harsher her temper gets and the harder she tries to push everyone away. After finding out she got pregnant she was shattered, so her behaviour became very angry, rude and overbearing, especially to Shinji whom she tried to push away as hard as she could precisely because he was the one could get through her.
The One I Love Is: Shinji. Several times. When Asuka tried -clumsily- to seduce him he thought -for absolutely NO reason- she was just trying to screw Rei, so that he slapped her, called her names and treated her like dirt during several days. Later he insulted Rei and made her crying because she had not told him who was the Fourth Child. In his defense, he later admitted he behaved like an ass and apologized to both girls.
Well to be fair, Flonne isn't there most of the time, making him a complete asshole to most people.
He also REALLY hates Adell and Mao for being main characters in the sequels to Disgaea.
In Herding Cats, Tavros of all people becomes a huge dick after being successfully shipped, particularly towards Equius, Sollux and especially Aradia to the point where the two of them become kismeses. It happens as a result of his becoming Vriska's moirail, because she helped him become confident enough to overcome his own problems and be more assertive, but neither realised how over the top he was being. Everybody else has to point out that there's a difference between countering light insults from others and being absolutely horrible to them. They also have to point out that anyone who does something as horrible as blaming Aradia for his spine getting broken and tries to play it off as being assertive seriously needs a reality check. He gets put through the wringer for it.
This actually happens to more than a few people. After Eridan and Terezi get together, Karkat flips his shit and goes off on an angry rampage that lasts for most of the fic. In turn, Terezi continually throws horrible barbs at him, coming off as a huge bitch herself. Later, Tavros rubs off on Gamzee, who continually treats Equius like shit.
Moonpaw in the Warrior Cats fanfic, Moonlit Path Disloya L Hearts. She starts out spunky with a bit of sass. Later on, she becomes a selfish jerk. Her sudden dip in niceness is more understandable when you consider that she started going bad right after she receives a dream predicting her death.
In the story Sylvia the Sylveon, Alseides and Vapor, two of Sylvia's brothers, the former due to the disappearances of two of his other brothers (both of whom, coincidentally, Took a Level in Kindness) and later, the illegal fighting for which he is trained after he himself is kidnapped, and the latter due to both being mistreated by his trainer and not getting the evolution he wants.
Navarone becomes increasingly unpleasant over the course of Diaries of a Madman, though this is acknowledged in-universe and also justified due to the bad stuff that happens to him over the course of the story. However, he does improve on this in a few areas as well.
Neville becomes more and more of a Jerk Ass Stu as Dumbledore's Army and the Year of Darkness goes on. Mostly this is due to the fact that he starts to see himself as a military leader and begins to expect military discipline from his underlings, while the said underlings still see him as their fellow classmate.
This happens in some Harry/Ginny fics, which emphasize Harry and Ginny's relationship, in part, by playing this trope straight with Ron and Hermione, despite the fact that both Ron and Hermione supported Harry/Ginny in canon. Particularly seen in the works of sbmcneil and Epeefencer, to the extent that Ron can sometimes border on Ron the Death Eater, and Hermione could greatly benefit from sensitivity training — from canon!Ron.
Prototype: Equestria Strains has Pinkie Pie. She tries to hide it under her normally happy self, but it's clear the stress caused by the outbreak and losing her friends is taking its toll on her.
Chrono Reflect has Standard Fluttershy. This is justified, though: After her best friend and sworn protector Rarity dies, she's left with no one to save her from her bullies. This forces her to stand up to her bullies, but she lets her drive to never be weak again take things to far and she ends up being the biggest bully of Canterlot High herself. Fortunately Forest Fluttershy forces her to take a level in kindness after beating her, and she starts being nice again. ...Somewhat.
In Love the One You're With Vanessa callously dismissed Gretchen as a “step down” for Ferb, effectively ensuring his crush on her is long gone.