When the play begins, the relationship between Jason and Medea has already turned sour - and it goes on to develop into a full-blown mutual antagonism, with tragic, indeed horrifying consequences.
At the start of the play we learn from Medea’s old Nurse that Jason has repudiated his marriage with Medea in order to marry Glauce, daughter of Creon, the king of Corinth. It seems he has done this solely to further his political standing in Corinthian society. Medea has come to seem an obstacle to him as she is a foreigner who is not readily accepted by the Corinthians and so he has callously put her aside. From this we can assume that he never really loved her. Medea, on the contrary, gave up everything else for his sake, leaving behind her country, her family, and helped him many times, because she was so in love with him; and his rejection of her leaves her desperate. The Nurse notes soberly notes how bad things have become, likening their relationship now to a sickness:
Now they're enemies.
Their fine love's grown sick, diseased, for Jason,
leaving his own children and my mistress,
is lying on a royal wedding bed.
Medea is an intemperate character who gives full rein to her passions - the Nurse remarks that ‘her mind thinks in extremes’; Jason on the other hand appears to be of a colder and more calculating disposition. The stage is set for a violent and all-consuming clash between the two.
At first, it is easy to sympathize with Medea as the wronged woman, left without husband or home in a foreign country. The Nurse, however, also fears for Medea’s children by Jason, recognizing that Medea puts her own passions before motherhood. She is also afraid that Medea will take some other extreme action like killing Jason and his new wife. The Nurse’s opening monologue foreshadows the later, wholly grim events of the play.
However, at first Medea does try to appeal to Jason, reminding him of all she’s done for him and how lost she is without him. When this has no effect, she sets about putting her plans for vengeance into action, and does not stop until she has murdered Glauce, Creon and her own children by Jason in order to exact the fullness of revenge upon him. She shows no remorse at all, as all her former passion for Jason has now turned into the most virulent hatred. Jason, for his part, is no less antagonistic in their final scene together. They hurl venomous accusations at each other, blaming each other most bitterly for all that has happened:
Jason: O my children,
you had such an evil mother!
O my children,
victims of your father's evil actions!
It is on this note of overwhelming anguish and hatred that the play ends. Euripides thus shows the most terrible results that a marriage gone wrong can have. Both Jason and Medea are guilty for the way that things turn out. It is Medea who performs the most appalling acts of murder, but it is Jason who sets up the enmity between them by his rejection of her.