How can Medea be defended for the crime of killing her children?I think the only route is by reason of insanity, but I'm wondering if there is more to it than I know at this point.

Medea is humiliated by Jason's new marriage and her pride will not quietly suffer this situation -- she is determined to seek revenge.  While it would be easy for her to kill Jason, she actually makes some sense when she decides to let Jason live the misery she can invoke!  She kills his new bride.  But she knows that that action is not going to go unpunished and that both she and the children are now going to be treatened by those loyal to Creon.  In Episode 5 she is vacillilating about the decision to kill the children, but part of her decision to do so is, in her mind, to protect them.  She says, "This shall never be,  that I should suffer my children to be the prey of my enemies insolence."  After knowing that the princess and Creon are dead she says, "as quickly as I may to kill my children, and start away from this land, And not, by wasting time, to suffer my children to be slain by another hand less kindly to them."  She is suggesting that it would be better to kill her children herself than to have them killed by her enemies.  While this does speak to her own pride, she is trying to justify her actions.  I don't think this makes the action defensible, but it makes some sense to Medea.  She does kill them to punish Jason, but it was not that simple for her.