To understand the Jason of "Medea", one must study him in the myth of "Jason and the Golden Fleece". In this story we learn how Jason was only allowed to take his throne after he had passed several tests of valor to prove himself. The final one was to rescue the golden fleece from Colchis. However, Medea (the daughter of the King of Colchis) is the one who actually accomplished all the tasks asked of Jason, and he took the credit for her help. Then, he moved Medea from her homeland, ditched her, and married the daughter of another King.
This allows us to see that essentially Jason is a weak man who allows others to do his work, and seeks to satisfy his own desires before those of others.