To start, it should be said that despite the innocence of many of his successful children's works, Roald Dahl is notorious for crafting creepy, even horrifying short stories. As other Educators have pointed out, as the story contains dynamic characters and events, the overall mood changes several times. However, if there were a term to identify the overall mood of the story as a whole, it would probably be ominous or foreboding.
At the very beginning of the story, we see a woman waiting innocently for her husband to return from work. Yet even at this early stage, Dahl drops hints to the reader that something is amiss. Mary Maloney is not simply "peaceful," but "curiously peaceful." Already Dahl casts doubts about her true state, implying that we should be suspicious, "curious" about her seemingly calm disposition. Also, "Her mouth and her eyes, with their new calm look, seemed larger and darker than before." This depicts a rather unsettling change. Dahl chooses "larger" and "darker" to portray her features, as though her eyes have become like holes.
When Mary Maloney's husband does come home, the narrator describes the reasons she adores her husband, one of which is that he doesn't complain about being tired. Then, immediately, her husband states: "Yes," he sighed. "I'm thoroughly exhausted." Again, something is not right here. The husband's actions stand in an unsettling contrast to the wife's dreamy, hazy thoughts.
All these ominous clues in the text lead up to Mary Maloney's confrontation with her husband. Evidently, he tells her he is leaving her (rather callously), and she becomes shell-shocked at the news, moving about numbly. Then, brandishing a frozen leg of lamb, she murders him. Immediately afterward,
The violence of the crash, the noise, the small table overturning, helped to bring her out of the shock. She came out slowly, feeling cold and surprised, and she stood for a few minutes, looking at the body, still holding the piece of meat tightly with both hands.
Yet the creepiness, the tension is still not fully released. We wonder, what will she do? How will she cover it up? What will become of her unborn child?
Mary Maloney pulls herself together, attempting to cover up her grisly action with a veneer of cheer. She practices in the mirror: "That was better. Both the smile and the voice sounded better now." She then proceeds to have a perfectly pleasant and mundane conversation with a shopkeeper while her husband lays murdered by her own hand. The juxtaposition of the friendly chat and the murder is sinister.
Tension continues to build and release a bit, build and release a bit, as Mary "discovers" her dead husband, the police arrive, and the reader realizes the weapon is currently cooking in the oven when Jack Noonan says "Get the weapon, and you've got the murderer." We continue to have apprehension about exactly how this whole drama will play out.
The unanswered questions and dramatic irony contribute to the foreboding atmosphere of the story right up until the end, when "in the other room, Mary Maloney began to laugh."