Why do mother's commit maternal filicide?

Today in class, we read an article on the topic of mothers who had shamelessly killed their children. Why did they do this? I believe this problem lies with mental illness. No mother nor parent, in the right mind would ever have the thought of killing their children, or in this case, actually pursuing too.
These mothers committed maternal filicide due to the fact they were sucicidal, experiencing depression or was mentally unstable. As said by Lanny Berman, executive director of the American association of sucicidology, Mothers don't kill their children unless they are seriously disturbed. She had also stated "If there is indeed no suicidal behavior on her part, then typically it’s homicidal behavior associated with a psychotic break or postpartum depression". Thus, I conclude, mothers who had murdered their offspring were mentally unstable.