Mary Maloney is definitely not thinking about murdering her husband at any time. She loves him and believes he loves her. She is just waiting for him to come home and looking forward to seeing him.
For her, this was always a blissful time of day. She knew he didn't want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house. She loved to luxuriate in the presence of this man, and to feel--almost as a sunbather feels the sun--that warm male glow that came out of him to her when they were alone together.
Tangible proof that she was not thinking about killing her husband is the murder weapon. Who would ever plan to commit a murder with a leg of lamb, frozen or otherwise? The deed was spontaneous, unpremeditated, almost unconscious.
"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.
Accordinig to the eNotes Introduction in the Study Guide, the story was written in the early 1950s. World War II had ended in 1945, and Americans were on a buying spree because of the nation's prosperity and the fact that there had been little to buy during the war years. The auto makers were not offering new cars to the public. Television had been invented, but the manufacturers were holding back on introducing it to the American public until the war ended.
One of the new items being introduced to the public after the war was big frozen food lockers five or six feet long and about four feet tall. People were buying huge amounts of meat because they could save a lot of money through buying in quantity. These big lockers lost their popularity for several reasons, one being the fact that if there was a power outage all the meat could be ruined and have to be thrown out. Another was that mammoth refrigerators with big freezers also came on the market.
Roald Dahl undoubtedly got the idea for "Lamb to the Slaughter" because of the introduction of these big frozen food lockers, most of which have vanished from the American scene. Other authors thought of stories in which someone kept a dead body frozen, since the lockers were roomy enough to accommodate a corpse. There was a small window of opportunity for stories of this sub-genre. Dahl's perfect-crime tale was not intended to be taken too seriously. Mary Maloney's sudden outrage is understandable, but she does not seem like the kind of woman who would commit a murder regardless of the provocation. That is one of the main reasons all the investigators never suspect her.
It is ironic that Mary is married to a policeman and that his murder brings an unusually large number of fellow officers to his home. Their long search of the premises for a murder weapon is part of the black humor, not only because they would never think of the weapon being a frozen leg of lamb, but also because they become hungry enough to eat the evidence.
Dahl might have considered having Mary plan the murder ahead of time, but it was the fact that it was not planned that kept her from being suspected. If she had planned to murder her husband, she would never have thought of using a frozen leg of lamb. Who would think of such an idea except Roald Dahl?