The mood of Lamb to the Slaughter changes quite dramatically as the story progresses. The peaceful, middle class domestic scene at the start of the story creates a mood of calm and tranquility. However, the suspense builds as we sense the husband Patrick's innate hostility to his wife. Later in the story the mood is particularly suspenseful as the police investigate Patrick's murder and we wonder if the calculating housewife Mary will be able to conceal the truth.
By the resolution of the story the mood becomes almost comical as the protagonist delights in the fact she has deceived the police into consuming the evidence, "She wants us to eat it. She said we ought to eat it up......And in the other room Mary Maloney began to laugh" (p. 4).
The title of Roald Dahl's story is a humorous allusion to the Bible, in which there are many places in which a man who is going to be killed is being led quietly and passively "like a lamb to the slaughter." The most frequently quoted is in Isaiah 53.7:
He was oppressed, and he was afflicted, yet he opened not his mouth: he is brought as a lamb to the slaughter, and as a sheep before her shearers is dumb, so he openeth not his mouth.
King James Bible "Authorized Version"
The allusion is humorous, but it can be considered appropriate in several ways. Obviously, Patrick Maloney has no idea what is going to happen to him, any more than a sacrificial lamb being led to the slaughter. Also, obviously, he is killed with a frozen leg of lamb. How could anyone anticipate that? Then there is the fact that Patrick speaks very little. He is the strong, silent type. When he does speak to Mary at length about wanting a divorce, most of what he says is not directly quoted but only inferred from the effect it has on his wife. So it would not be inappropriate to say of Patrick that he "opened not his mouth." It is especially true of him that he "opened not his mouth" about his growing dissatisfaction with his marriage and the plans he was making to obtain a divorce. It is to Mary's advantage that Patrick "opened not his mouth" to discuss his marriage with any of his colleagues on the force. None of the policemen have the slightest idea that Patrick and Mary are not happily married. Consequently, there is no cause to suspect her of killing him. The title is also appropriate because the whole story is based on the bizarre idea of using a frozen leg of lamb as a murder weapon and then serving it cooked to the detectives investigating the case. Many lambs are slaughtered by men, but this is a unique situation in which a man is slaughtered by a lamb. Mary herself has been a "lamb" all her life until her understandable reaction to her husband's cold-hearted rejection when she is six months pregnant.
The story opens with a very tranquil scene. We see a woman, who is six months pregnant, waiting in anticipation for her husband to come home. She deeply loves her husband and when he arrives she is excited to see him. She rushes around, happy to fix him a drink, and tells him if he too tired she will just cook dinner for him at home instead of going out. The mood then shifts drastically when her husband, Patrick, a police detective, tells her some shocking news. We the reader are not privy to what he has to tell her, but by her reaction we know it is bad news for her. She is in shock and almost goes into a trance, and then decides to cook dinner. She goes and gets a leg of lamb from the freezer, and uses that to kill her husband, then cooks the lamb.
We see that there are many moods in the short story: first we see a happy mood, then it shifts to a mysterious mood. As the other police officers come to investigate the murder, the mood shifts again, to a mood of being almost sadistic. As the police officers are eating the leg of lamb, they are talking about trying to find the murder weapon, while the whole time, they are eating the murder weapon. The story shifts to the wife, in the other room, listening to the police, and she starts to laugh to herself.