You can be bitchy, but don’t be evil. There was no limit to the amount Courtney would chastise her classmates, whether they were friends, boyfriends, or enemies. Much like Tracy Flick, she charged forward without glancing at the wreckage. She didn’t care much about her dead friend, and she didn’t consider the consequences when she changed Fern into Vylette, and gave the quirky girl a newfound sense of power and cockiness. Naturally, destroying Vylette would make young Fern hungry for revenge. She teamed up with the scorned and grieving Julie and served up revenge that was epic and status-demolishing rather than swift and easily forgotten.
The cinematic jerks never seem to learn. Hilary Faye scorned the sweet Mary for having sex in Saved!, until all of that pent-up Christian anger came back to haunt her (see also Easy A). Basically, if you take meanness to new heights, it will come back to haunt you.