Patrick Maloney dies early in the story. Judging from the way he talks to his wife before his death, he does not treat her unkindly but with cold indifference. An example of his manner towards her can be seen just after he has informed her that he wants a divorce.
"So there it is," he added. "And I know it's kind of a bad time to be telling you, but there simply wasn't any other way. Of course I'll give you money and see you're looked after. But there needn't really be any fuss. I hope not anyway. It wouldn't be very good for my job."
Roald Dahl carefully refrained from making the husband a brute and also from making his wife seem abused or the least bit angry or unhappy. It is important to the plot that no one would know of any friction between husband and wife. Patrick himself says he wants to avoid "any fuss." Dahl establishes that the husband still comes home from work at exactly the same time every evening. It does not appear that he is having an affair with another woman or is giving his wife any other kind of cause to dislike him, much less to want to murder him.
This is a perfect-crime story. Edgar Allan Poe's "The Cask of Amontillado" may have been the first such story. In any case, it is the best. Montresor, the first-person narrator, states that he wanted to be above suspicion when Fortunato disappeared.
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued as was my wont, to smile in his face, and he did not perceive that my smile NOW was at the thought of his immolation.
When someone gets murdered--especially inside his or her own home--the first person the police suspect is the spouse. This is why Dahl takes pains to establish that, as far as the outside world is concerned, the Maloneys are a happily married couple.
Dahl also takes pains to establish that Mary Maloney is a meek, loving, gentle homebody who would seem totally incapable of bashing a man's head in with a blunt instrument. The fact that her husband is a cop works in her favor, because he could have made many enemies in his years on the police force. And the fact that he was a cop brings an unusually large number of men to his house to investigate the crime. They also spend many hours there because Patrick was "one of ours," which gives Mary time to cook the frozen leg of lamb and serve it to a big group of hungry men.
Mary's sudden change of character is surprising but not unbelievable. The reader understands that many people who appear meek and mild may be capable of just such spontaneous outbursts as Mary exhibits when she uses the frozen leg of lamb as an unconventional instrument of murder.
The story was published in 1953. In those days many Americans were buying home freezers that were so big they typically had to be kept in the garage. Supposedly the owners could save money by purchasing large quantities of meats at wholesale prices. But these freezers lost popularity. One reason was that the electricity they consumed detracted from the putative savings. Another reason was that power outages could spoil an entire freezer full of meat. And, as suggested in "Lamb to the Slaughter," big cuts took a long time to thaw and may not have tasted as good as fresh meat. It was a cute trick to have Mary use the leg of lamb as a club, but then she was faced with the problem of dethawing it before she could destroy the evidence by feeding it to the very men who were searching for the murder weapon.