Why do mothers commit the unthinkable act of maternal filicide?

Mothers commit the unthinkable act for certain reasons. Some parents think it is in the best interest of their child, others claim they hear voices. Some don't even mean to and just abuse them, while others believe they are a hindrance in her life. In very rare occasions, the mother kills the kid to get back at the father. I relate this last one to Medea because to get revenge on Jason, she kills her own children. Although it may always be difficult for a mother to kill a child, some find it within themselves to go through with it. All of these mothers that commit the unthinkable are usually mentally ill. In the story it says "Mothers don't kill their children unless they are seriously disturbed." Although this is the case for most, I don't think this goes for the same with Medea. I think this because although Medea committed the unthinkable, she was sane when she had the moment of determining whether to commit the act or not.