She's the one who ratted out Ned's cout to Cersei in the books, because she couldn't stand not having her fairy tale life in the capital: (spoilers) Little Finger should have pushed her ass through the moon door. Everything Joffrey did to her was nothing compared to what happened to Ned; and it was entirely HER fault! She was an asshole before Ned's beheading, an asshole during her entire time at king's landing; and now she's a homicidal asshole (spoilers) who's going to kill her anemic cousin to become queen of the vale.
The show conveniently overlooks that ugly little piece of GOT canon: because then we'd be bored and angry every time she's on the screen. It's just another change from the original story that apologizes for horrible people. HBO just can't leave well read enough alone.