Medea is the original Femme Fatale. She's a sorceress who is most famous for helping Jason and the Argonauts steal the Golden Fleece. After that she marries Jason, who later ditches her for the princess of Corinth. So, Medea does what any reasonable person would do and kills the princess, the princess's father, and her and Jason's kids.
When we meet Medea in the story of Theseus, she's taken refuge from a lot of very angry Corinthians by marrying King Aegeus. She's super threatened by Theseus, because she knows that Aegeus will name the young hero as his heir instead of her son Medus. So, in typical homicidal Medea-style, she attempts to kills Theseus.