The tone of “Lamb to the Slaughter” is a surprisingly lighthearted one given the subject matter. Dahl toes the line between humor and horror, often landing on the side of humor, as seen in the following passage (which comes after Mary impulsively hits Patrick on the head with a frozen leg of lamb):
The violence of the crash, the noise, the small table overturning, helped bring her out of her shock. She came out slowly, feeling cold and surprised, and she stood for a while blinking at the body, still holding the ridiculous piece of meat tight with both hands.
All right, she told herself. So I've killed him.
Here, Dahl uses a more earnest tone when describing “the violence of the crash” and the way Mary feels “cold and surprised” by her actions, only to undercut the gravity of the moment by having Mary nonchalantly think, “All right. So I’ve killed him.” The lighthearted tone here communicates to readers that Dahl is less interested in exploring the psychological effects of murder on Mary and more interested in setting up the darkly funny events that unfold after the murder, including Mary (very easily) convincing the police that she is innocent and feeding them the leg of lamb that she used to kill her husband.