“Lamb to the Slaughter” is a short story that belongs to the genres of black comedy and horror. Fiction in the black comedy genre uses humor to explore serious and heavy topics like death, violence, and loss. In this story, Dahl makes Patrick’s violent death comical by having Mary kill him with a frozen leg of lamb and then serve the lamb to the detectives investigating his murder (a humorous example of dramatic irony).
This story can also be considered an example of horror, given its haunting and grotesque elements. Take the following passage, for example, in which Mary—who is pretending she does not know how Patrick died—persuades the detectives to eat the murder weapon:
"Please," she begged. "Please eat it.” […]
There was a good deal of hesitating among the four policemen, but they were clearly hungry, and in the end they were persuaded to go into the kitchen and help themselves. The woman stayed where she was, listening to them speaking among themselves, their voices thick and sloppy because their mouths were full of meat.
The unsettling nature of Mary’s feigned innocence here, alongside the somewhat repulsive description of the detectives’ voices being “thick and sloppy because their mouths were full of meat,” make this scene, and the story as a whole, a prime example of horror.