Mary Maloney, Roald Dahl's character and the title of his story, "Lamb to the Slaughter," are telling components of this murder scene. Mary is the very controlled and convincing wife, who kills her husband, after learning that he is about to leave her. If Mary were to be charged with her husband's murder, she would be found Sane and Guilty of Murder in the First Degree.
Mary realizes that her marriage is in trouble, and has known it for some time. She knew this day would come, she has resolved it in her mind, and that is why she is able to conduct herself in a "tranquil" manner. Mary planned the scene, it's ten to five and the curtains are already drawn. This is, of course, so no one will see her commit the crime. She sees to it that her husband has had a couple of whiskies, and that he is not as alert as he should be.
She later gives the police investigating the crime some whiskey, so that they too are more relaxed and not as aware as they should be. After learning that her husband is leaving her, Mary is unemotional, perhaps she is in denial and proceeds to begin cooking dinner. Mary then kills her husband with a frozen leg of lamb, when he refuses to eat with her. After the murder Mary simply states, "so I've killed him." Some might say that she was "insane" at the time of the murder, but an insanity defense is not often used, and when it is used, it is frequently unsuccessful.
Mary covers up the slaughter perfectly. She cooks the leg of lamb, cleans herself up and goes to the local market looking for peas and potatoes. She tells the owner she is cooking...