Medea killed Pelias (or drove his daughters to kill him) because he refused to relinquish the throne of Iolcus, meaning that he stood in Jason's way and hers. This is one of several events in Medea's bloodthirsty career before she ever set foot in Corinth, all of which would have been familiar to Euripides's audience. Before killing Pelias, she had butchered her own brother, Absyrtus, and thrown his dismembered body in the sea to slow down her father's pursuit of the Argo. These events make it clear that Medea sets her own standard for ruthlessness and barbarism, making her murders during the course of the play quite predictable.

Jason's nature is more open to interpretation than Medea's. She has been fiercely loyal to him, whereas he, at the beginning of Euripides's play, has betrayed her. Medea is utterly without mercy or compunction when it comes to her enemies. She was fanatically loyal to Jason until he made an enemy of her. Whatever happens to him after that, from Medea's perspective, is his own fault. However, it is quite possible that Jason, a much weaker and less ferocious character than Medea, is appalled by the murders of Pelias and Absyrtus and that this is one of his reasons for leaving her. Whatever motive he has for abandoning Medea, her past treatment of everyone who has stood in her way makes it very clear that Jason is doomed as soon as he betrays her.