Ms. Bitters

MIss Bitters is a cruel, sadistic, and depressing elementary school teacher in the Invader Zim series. She tries to keep her class as miserable as possible

Her past is never explained in the series.

She moves with serpentine movements.

While not really an antagonist she qualifies as a villain. All her lessons usually involve promoting villainous acts, such as canibalism, or stating howsociety or the universe will eventually collapse on itself.

In the episode Tak:The Hideous New Girl, she sent two classmates to their doom just to make room for Tak to sit. One of them was sent just because she was tired of him.