Mary Maloney succumbs to a fit of rage and kills her husband with a frozen leg of lamb which she happens to have in her hands.
At that point, Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.
This might be called temporary insanity. But on the other hand, perhaps most of us are capable of doing something comparable in extraordinary circumstances. And most of us, presumably, are sane. Otherwise, Mary Maloney seems completely sane. She is a good homemaker and loves her husband. She is looking forward to having a baby. After she has done the deed and her husband is dead, she shows perfect rationality in looking out for herself and her unborn infant.
It was extraordinary, now, how clear her mind became all of a sudden. She began thinking very fast.
The omniscient third-person narrator frequently goes inside Mary's mind and never discovers any evidence of insanity. Mary establishes an alibi and gets the investigating policeman to destroy the murder weapon by eating it. All of Patrick Maloney's colleagues, who have known his wife for a long time, regard her as a perfectly normal woman and a law-abiding citizen. She has always been such a loving and devoted wife that no one could suspect her of murdering her husband.