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This is an excellent short story by Roald Dahl that is normally taught to explore how irony operates in literature. However, the inciting incident, which brings about the central murder of Mary Maloney's husband, comes when Mary is told a terrible bit of news by her husband. Note how it is introduced:
And he told her. It didn't take long, four or five minutes at most, and she sat very still through it all, watching him with a kind of dazed horror as he went further and further away from her with each word.
We are never told specifically what it is that he tells his pregnant wife, but it is obvious that it is something that means he will be leaving her with her unborn child and moving on to some form of different life, as suggested by the way that "each word" seems to take him away from her. This is what leads to Mary's sudden and surprising act of clubbing her husband on the head with the frozen leg of lamb.