How we cite our quotes:
Jason: Anything you or the children want in exile,
let me know; I'll gladly furnish it […]
Medea: The presents of the wicked are pure poison. (58)
Jason seems to feel guilty about the way everything is going down. By denying his help in exile, Medea keeps him from easing his conscience. It's yet another way that she gets revenge on her husband.
Leader: But, my lady, to kill your own two
Medea: It is the supreme way to hurt my husband. (140-141)
Medea's hatred for Jason is so fierce that she'll go to any lengths to hurt him. She feels that her revenge wouldn't be complete if the boys are left to live. By the end of the play, Jason is destroyed.
Jason: But, Medea, what is this --
these dewy eyes, these tears; […]
Medea: It is nothing.
I was just thinking of our sons. (150-151)
It's open to interpretation as to exactly why Medea is crying here. They could be fake tears, meant to convince Jason of her sincerity. She could also be lamenting the fact that she's going to murder her sons in a very short period of time.