Heathers (1988)
A black comedy about a clique of similarly-named girls who rule their high school through intimidation and sex appeal, the Heathers are rich, pretty, and popular — but they are feared more than admired.
Veronica, a member of the group who is growing disillusioned by the Heathers' cattiness and sense of entitlement, makes an unlikely bully-avenger. After a drunken fight, Veronica (with friend J.D. as an accomplice) accidentally gives one of the Heathers a glass of toxic drain cleaner, which kills her. They frame the incident as a suicide and get away with it.
Feeling empowered by their manipulation J.D. and Veronica shoot two male bullies in another act of revenge. Veronica, though, feels they have gone too far, and begins to recognize J.D.'s mental instability. J.D. is actually plotting to blow up the school at a pep rally, and after Veronica confronts him, he ends up killing himself.
The movie presents a variety of bullies: the superficial cruelty of the clique, the Veronica's bully-victim, and the truly disturbed victim, for whom any level of retaliation is justified.