One technique that R. Dahl employs in "Lamb to the Slaughter" is referred to as black humor:
Black humor is the use of the grotesque, morbid, or absurd for darkly comic purposes.
Mary Maloney is pregnant enough that she moves slowly. She welcomes her husband home, from work—a policeman—ready to wait on him hand and foot. Even though his wife seems to be in her latter stages of pregnancy, he decides that this is the best time to tell her that he is leaving her, in a very matter-of-fact way...worried more about himself than his wife, who is moving ever closer to giving birth to their child:
And I know it’s kind of a bad time to be telling you, but there simply wasn’t any other way...I’ll give you money and see you’re looked after. But there needn’t really be any fuss...It wouldn’t be very good for my job.
Something in Mary snaps. She goes into the basement and brings up a leg of lamb, a common main course in the 1950s (when this story is written); he stands with his back to her—at the window—telling her he doesn't want anything to eat, but dinner is not what she has in mind: she brings the frozen meat down on his skull hard enough to kill him.
This, however, it not the occurrence of black comedy. First Mary cooks the meal. She has taken care of details to supply her with an adequate alibi, having visited the grocer for the rest of the things she needs for dinner, while her husband's corpse rests on the floor in the livingroom of their home.
Mary arrives home, as if the murder occurred while she was out, calls the police and they set to work. As the meal is finished, the policemen are tired and hungry, so she encourages them to have a drink and sit down to dinner.
This is the moment of black humor: she feeds the murder weapon to the police. This is also an example of irony.
[I]rony of situation is a discrepancy between the expected result and actual results.
Irony of situation is the other major technique Dahl uses. For certainly, the reader does not expect that the police would not only be consuming the murder weapon, but also the only piece of evidence that could possibly implicate Mary. So this segment of the story is not only ironic, but also serves to further the intent of the author's use of black humor. The murder weapon (as one policeman surmises) is right under their noses, though they are completely unaware.
One of them belched.
“Personally, I think it’s right here on the premises.”
“Probably right under our very noses. What you think, Jack?”
And in the other room, Mary Maloney began to giggle.
This classic short story always seems to elicit a creepy kind of surprise, and perhaps that is why it is highly anthologized, even a favorite story in high school classrooms; in our district, it is on the final, and an excellent choice because it is so compelling—grabbing the reader's interest until the very end.
Lamb to the Slaughter" demonstrates Dahl's fascination with horror (with elements of black comedy).It is told from the point of view of the murderer, Mary Maloney and this aspect is the most interesting aspect of the story.The reader knows only what she knows, which means that the reader knows more than the other characters which is quite uncommon and is called THE DRAMATIC IRONY and also the reader is not given access to reasoning behind Patrick's leaving his devoted wife. SO here reader tends to favour the murderer naturally.Also , it has a dark humour where lamb is used as a weapon which twists thhe meaning of lamb to the slaughter into someting which is not a metaphor but somethinggg that actually happens