"Lamb to the Slaughter" is told from the point of view of Mary Maloney. The story starts off when she is all alone, so we have to be in her point of view at the start. Her husband's arrival, his mood, his behavior are all told through Mary's point of view. In fact, the story is largely based on her over-attentiveness, her close observation of her husband, her concern about his health, his state of mind, his appetite, his job, his drinking, and everything else about him. Patrick can't stand all this attention, all this scrutiny, all this mothering. This is what has made him decide that he wants a divorce. This in turn leads to Mary clobbering him over the head with a frozen leg of lamb.
It is important that the story is told from Mary's point of view because this enables the reader to understand why she acts as she does. Being in her point of view forces the reader to identify with her, in spite of the fact that she has committed a murder. We understand her fit of rage as a reaction to her extreme devotion to her husband and his cold rejection of her when she is six months pregnant. We understand that she doesn't want to be caught and punished. We understand her thought processes as she goes about concocting an alibi and getting rid of the evidence. We are, in fact, accessories after the fact.
The other characters are only minor ones except Patrick, who is not even a very important character himself. To narrate part of the story from the point of view of one of the policemen would disrupt the narrative flow. It is always disruptive when a writer switches from one point of view to another. To switch from Mary's to Patrick's point of view would not only disrupt the reader's identification and involvement, but it would probably generate some sympathy for Patrick which would detract from the reader's sympathy with Mary. And the author wants the reader to sympathize with, and to identify with, Mary from beginning to end. An example of how Roald Dahl avoids going into Patrick's point of view is the way he has Patrick tell Mary all his feelings about their marriage and his decision to get a divorce.
"This is going to be a big shock to you, I'm afraid," he said. "But I've thought about it a good deal and I've decided that the only thing to do is to tell you immediately." And he told her. It didn't take long, four or five minutes at most, and she sat still through it all, watching him with puzzled horror.
We don't hear a word of Patrick's case. We can only imagine what he is telling her from her reaction, i.e., from her point of view.
...and she sat still through it all, watching him with puzzled horror.
Once Patrick is dead there is no feasible point of view other than Mary's. She has a dark secret. We alone know she is guilty and want to see her get away with her crime. The story proper really begins here. It is about how a woman commits a perfect crime by killing her husband with a frozen leg of lamb and then getting the investigators to eat the lamb after she cooks it. We have to know what she is feeling, thinking, and planning. And we have to believe in the dramatic change of character that took place in this passive, humble little housewife.