The most important event in "Lamb to the Slaughter" must be Mary Maloney's sudden impulse to kill her husband with a frozen leg of lamb.
"For God's sake," he said, hearing her, but not turning round; "Don't make supper for me. I'm going out." 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.
She might as well have hit him with a steel club.
Mary's sudden change of character is startling. Up to this point she has been portrayed as a soft, loving, nurturing wife.
Heaven has no rage like love to hatred turned,
Nor hell a fury like a woman scorned.
-William Congreve 1670-1729
Dahl does not relate exactly what her husband Patrick had just told her, but it is obvious he had said he wanted a divorce. Everything else in the story proceeds from the murder.
Mary is concerned about her own welfare as well as that of her unborn child. If she went to prison for manslaughter, what would become of the baby? Being a cop's wife has taught her a lot about police procedure. She knows that she must dispose of the murder weapon and establish an alibi. The story shows that Dahl has given a great deal of thought to all the details. Even with the oven turned to its highest temperature, it will take hours for the leg of lamb to thaw out and be properly cooked. Mary goes to the grocers and spends some time there in order to establish that she was not at home when the murder occurred. Then when she calls the police her house is swarming with investigators who are especially concerned about finding the murder weapon.
Because Patrick Maloney was a policeman himself, the investigators spend far more time at the scene of the crime than they would have spent on an ordinary homicide. They all knew Patrick, and most of them know his wife. It never occurs to them that she could have been resposible for the crime because she is known to be such a devoted wife. The time element is important. It is going to take several hours for that leg of lamb, the murder weapon, to be thoroughly cooked. The police are willing to spend the time on the case because they will always give special attention to a "cop killing."
In cases of domestic murders, the spouse is usually the prime suspect. If the police knew that Patrick wanted a divorce they would have been more suspicious of Mary. But Patrick is portrayed as the strong, silent type who keeps his thoughts to himself. Evidently there is no question of his being involved in an illicit love affair. Dahl makes that clear when he shows that Patrick always comes straight home from work and presumably stays home.
When the clock said ten minutes to five, she began to listen, and a few moments later, punctually as always, she hears the tires on the gravel outside, and the car door slamming.
Mary manages to commit the perfect crime because she has an unblemished reputation as a devoted wife, because she is six months pregnant, proving, among other things, that her marital relationship must have been satisfactory, because she is a cop's wife and knows a lot about police procedure, and because she happens to be holding a frozen leg of lamb when she is overcome with pent-up rage.