Andrea Yates had been taken off her Haloperidol when she drowned her 5 children. She too was ridding them from evil,being another religious person,so thought by killing them that she'd free them of the evil that they " possessed ".
It's definitely the withdrawal/reduction of these powerful drugs which causes the patient to kill,as they become more and more agitated and their thoughts are skewed.
Any medication,whatever it is, once it's stopped, can cause symptoms of an illness to return with a vengeance,and for what little that was left in Sheila's system,would have caused her inner problems.