This short story by Dahl crosses a very fine line between two genres, that of comedy, more in the sense of black humor, and that of crime fiction. Mary "giggles" at the end of the story when she has actually succeeded in getting away with the murder. Of course, the crime is the killing of her husband, Patrick, with the frozen leg of lamb. The reader is forced to see the humor in the situation as the murder is investigated and the detective is stumped, for how bizarre would it be to discover frozen meat as a murder weapon?
Mary outsmarts the detectives and is absolved of the crime. The absurdity is highlighted by the title of this work, "Lamb to the Slaughter." Ironically, one must question whether Patrick is also the metaphorical lamb which would make Mary's choice of murder weapon, the leg of lamb, even more ridiculous!