"A family formed by crime must be broken by more crime."
In his outline of the plot, Seneca emphasizes how Medea and Jason effectively side-step decorum in the establishment of their relationship. Jason uses Medea to sustain and protect himself, benefitting from her arts. By the time she enacts her revenge she has proven that a relationship formed for the benefit of a single party cannot end with that party's continued benefit.
"Poor woman, do you want to know where hatred ends? Look to love."
Medea is consumed by hatred because in it she finds a stronger master than fear, which is what Jason inspires in her when he leaves. She has few resources upon which to draw, so she diverts them all to a focused goal -- revenge. If only she appealed to Jason and her new neighbors through love, she may have discovered a yet stronger master.
"The one who knows no hope knows no despair."
In other words, Seneca believes that desire is the origin of suffering. Medea hopes that Jason will return to her, so she despairs when he does not change his mind. She cannot see the whole picture, however, and acts out of a premature despair. Her hope became her undoing because in its absence she was left with only hatred and fear.
"When arrogant hands seize power, the ruler thinks authority resides in stubbornness."
This is the subplot of the play by Seneca. He explores how authority can be misused and thus misrepresented to the people. In the case of Jason, he attempts to win favor with Creon by marrying his daughter, despite his previous marriage to Medea. He is looking to engender himself graciously in the house of the king, in the hopes of one day stealing his throne. This action is also a reflection on Creon, not just Jason, because he runs his kingdom in a manner which demonstrates his own weakness of character and susceptibility to vanity as well as rewarding corruption.
