All the violence and terror in Medea is caused by Jason's betrayal of his wife Medea. Her sheer rage at his unfaithfulness drives her to commit horrific acts of bloody revenge. Ironically, Medea's fury at her husband's betrayal drives her to the use of trickery and manipulation, which are really just another form of betrayal. Medea shows how, when one person betrays another, all may be corrupted.
Jason's unfaithful behavior and lack of sensitivity towards Medea is symbolic of the overall unfair treatment of women.
Jason never betrayed his family or Medea; he only acted in their best interest by marrying Glauke.