A Brooklyn mother has confessed to smothering her son to death hours before she threw herself in front of a subway train, according to police.Police say the woman told them she was overcome by "demons" when she allegedly killed her son Monday in their Crown Heights apartment. The mother, who had been treated for depression, told investigators that the trouble began when her son woke up Monday morning still upset that a playmate didn't come to his family birthday dinner Sunday.Charges are pending against the mother. She is being treated for a broken leg and severed fingers at Lutheran Hospital. Related Information Child found dead in Crown Heights apartment, mother hospitalized