The Question and Answer section for Medea is a great resource to ask questions, find answers, and discuss the novel.
His main mistake is that he allows himself to be decieved by her pleas. If he had not given in to her she would not have been able to kill his fiance or his children
Jason is a famed adventurer and so you could say Medea gains some of his kleos. Medea did not marry him for material benifit though really, she was madly in love with him and chose to travel from Colchis to marry him, killing her brother on the...
Creon banishes Medea from the city because he fears she will seek revenge after Jason abandons her and remarries. Jason has married Creon's daughter.