Seneca's Medea is one of several versions of the story. He focuses not only upon Medea's revenge but upon the relationships to power which Creon, Jason, and even Medea demonstrate. He's eager to explore how the human psyche responds to temptation and betrayal. At every turn, he paints corruption. It's not a pleasant story, though it deserves telling.
After moving Medea, his wife, to Corinth following their flight from Crete, Jason promptly abandons her and their two sons. He starts pursuing King Creon's daughter, Creusa. When Medea learns that they are to be married, she loses her mind. Her hope previously had been that he would return to her and be a husband to her and a father to their children, but she cannot understand this blatant betrayal. She pursues a thorough revenge which culminates in her murdering the two boys in front of their father. For his part, Jason is seeking power by overthrowing Creon, fulling intending to instate Medea, not Creusa, as his queen. His methods, however, are not honorable and yield only pain.