Why did scylla turn into a monster?

Scylla, in Greek mythology, was transformed into a monster as a result of jealousy and betrayal. According to one version of the myth, the sea god Glaucus fell in love with her, but she rejected him. In a fit of rage, the sorceress Circe, who was spurned by Glaucus, poisoned the waters where Scylla bathed, causing her to turn into a fearsome creature with multiple heads, forever doomed to terrorize sailors.