Why do some mothers commit the unthinkable act of maternal filicide?

Some mothers commit the unthinkable act of maternal filicide because some moms have mental illness and that could be a reason they kill their their children. And most don't even believe that their own mother would hurt them because the is supposed to love and take really good care of their children and be  abusing them. This women Teresa caffese said if the mother never had a mental illness she would've never killed her children. Phillip Resnick, an expert in maternal filicide and a co-author of a 2007 World Psychiatry study, reviewed psychiatric studies on mothers killing their children and found most had depression, psychosis and suicidal thoughts.