i'm familiar with the case -- if it's the one i'm thinking of.
first off, "insanity" is not a psychological condition; it is a legal definition.
one can be quite mentally ill and not be "insane."
the insanity question is basically "did the person know what s/he was doing, and did s/he know at the time the crime was committed that the act was wrong?"
i believe that she DID know that what she was doing was wrong. again, if this is the case i'm thinking of, she demonstrated this. after killing one child and before killing another, her husband was waking up -- she went to him and made sure that he was sleeping again before going on to commit the next murder.
now, if she was really doing this because "god told her to," she would have NO REASON to be secretive about it. she knew that she was committing murder, and she knew that it was wrong -- otherwise, why would it matter to her if her husband might be waking up and so might catch her doing it?
i'm amazed that she was found to be insane.