Betrayal
When Jason leaves Medea to marry Creusa, he effectively abandons her and their children. Having brought Medea to this foreign country, he has established her in a vulnerable position and then leaves with no explanation. Naturally she feels betrayed. In response, she concocts this scheme to make Jason pay by betraying his trust in her as well. He trusts that she will remain a good, faithful mother. She proves, step by step, that she will do anything to exact her revenge, including sacrificing her stability as a role model and even sacrificing the children themselves. This is her ultimate reciprocal betrayal, taking Jason's loved ones from him the way he removed himself from her.
Superstition
The characters of this play are deeply attune to spiritual matters. They base their decisions on both emotion and superstition, but the superstition appears to carry greater weight. After all, Medea is first feared by her new Corinthian neighbors because of her reputation with the dark arts. She's a sorceress of high merit. When she receives an oracle, she interprets the event as a threat upon her own power. She refuses to leave Corinth in order to spare her unfaithful husband, despite the dangerous promise if she doesn't. Ultimately, Medea fulfills the prophecy by killing her own children, proving her emotions less powerful than fate.
Displacement
Medea's primary dilemma in this play is not so much her husband's disloyalty but her own displacement. As a foreigner, she is lonely and upset. Her entire world has been radically transformed by her love for Jason which led her to Corinth, but now that he has abandoned her she is painfully conscious of how lost she feels. When Creon threatens her with exile, he effectively confirms a secret suspicion -- she will never feel at home again. This displacement morphs in Medea's mind into a kind of desperation which corrupts her mind and decision making. She starts believing that she has nothing to lose and thus kills her sons and betrays her husband, fulfilling her own worst fear. In the end, she does leave Corinth.
