Joni from Boy Meets Boy is shown as a good, somewhat tomboyish straight girl and friend of gay teens Paul and Tony. But then she hooks up with dumb jock Chuck, who is really most likely dating her out of spite for a previous girl who spurned his advances, and things go downhill from there. Although her friends try to like Chuck, it's no good and then they don't try very hard to hide how much he annoys them. Even so, this doesn't excuse Joni for giving all of them (except Chuck) the silent treatment, not returning calls, or dropping completely out of their lives after about a decade of friendship. To her credit, she probably goes back to making nice with Paul and Tony in the end.
Due to sleep deprivation, Jade Harley of Homestuck has gotten a lot grumpier - although it should be noted that she's only like that to the trolls, who have been picking on her for her entire life (with the exception of Tavros). She got better later on, though.
Something similar happened to John in Act 6 due to Cabin Fever.
According to Aranea, Cronus wasn't always as big a Jerkass as he later became, but he was never exactly a nice guy, either. Apparently, the revelation that magic didn't exist - something he'd based a good deal of his life around - caused a Heroic BSOD leading to this trope.
And now, Aranea herself combines this with Face-Heel Turn, becoming not only more sadistic and arrogant, but also unbelievablysmug.
DiDi of Ménage à 3 originally appeared to be sweetly nice if ditzy, but her desperate desire for an orgasm eventually drove her to increasingly erratic and sometimes bitchy behaviour. Some readers, looking at her casual, careless treatment of her numerous boyfriends (and her later insanely unthinking treatment of tyhe frustrated Zii), suspect she was a bit of a Bitch in Sheep's Clothing all along, but it might be more accurate to say that her ditziness is actually nigh-autistic social and emotional incompetence.
Seymore of Sinfest became more and more of an asshole around 2010, culminating with the "Bad Behavior" series.