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.