Why do mothers commit the unthinkable act of maternal filicide?

Why do mothers commit the unthinkable act of maternal filicide? 

The article by The Washington Post, “Why do moms kill their children?”, explains reasons for a mother killing their child. However, studies and experts agree on one common factor when mothers commit such an unthinkable crime: extreme mental illness. Only the most insane and crazy mothers exceed to the level of murdering their own child. Maternal filicide is very rare, but each time it occurs, it is because the mother is seriously disturbed, said Lanny Berman, executive director of the American Association of Suicidology in Washington, D.C. A mother doesn’t just kill her child for no reason, there is always a reason behind it. A mother who threw her children into the San Francisco Bay did this terrible thing because she heard voices telling her to throw them in the water so they could reunite with God in heaven. Although this mother had a history of schizophrenia and other mental illnesses, she truly believed that it was the right thing to do and it would be better for her kids. Clearly we see that nothing is worse than killing your own children, but she had good intentions which unfortunately was not a good thing.