Why does Medea think it is necessary to kill her sons to get revenge on Jason?

I think that this particular action is where Medea ends up
moving into a realm where her actions are indefensible.  In her mind, she sees it
necessary to kill Jason's sons for a couple of reasons.  One of the reasons she uses to
justify killing the children is because of pragmatism.  Medea figures that "the
Corinthians will kill the children anyway, in retaliation for her murder of Creusa and
Creon."  This aspect of practicality in Medea is one reason she uses for her actions. 
Another reason is out of pure spite and wrath towards Jason.  When Jason indicates to
her that she will suffer as well, Medea argues that this experience is secondary to her
being able to take from Jason.  The anger that is felt towards Jason compels her to kill
the children.  In another respect, Medea represents the idea that the emotional
experience of jealousy and vengeance can be all encompassing, one that knows no
limitations.  Civil society would regard the killing of children as one of the worst
crimes and a point from which there can be no return.  Yet, Medea's jealousy and rage,
her feelings of hurt caused by Jason's action, causes her to be irredeemable and past
that point, demonstrated by the killing of her own children.