Actually, a very interesting question.

In the mid-80s, two brothers who were fundamentalist Mormons butchered the wife and two year old child of their third brother, because they received direct instruction from God to do so. Dialoging with God on an active and ongoing basis is a fundamental tenet of Mormonism. If God instructs you to tear down your garage and build a Temple to Him, you are duty-bound to do His will. God had running instructional dialogues with Joseph Smith, Brigham Young, and many others right up to the current leader of the Mormon Church. Blood atonement, the notion that certain sins can only be atoned for by the shedding of the sinner's blood, is also fundamental to Mormonism. Thus, if God's voice comes to you and tells you your brother's wife is a sinner and she needs to die for it, can you plead an insanity defense? Belief in the unseen and the irrational is part and parcel of most faiths. Is it insane only at the extremities of its practice?