In "Lamb to the Slaughter," how does Roald Dahl engage the reader?

Roald Dahl begins the story with a familiar domestic scene: a wife sits patiently sewing, waiting for her husband's return from work. When he arrives, the reader notes her loving and rather servile attitude toward him and his stereotypical "lord of the the manor" behavior as he pours a whiskey and fends off her offers of his slippers, cheese and crackers, or supper. The narrator makes us believe that Mary is nothing but a kind, gentle, and lovely pregnant young wife who loves her husband.
Readers become uneasy after Mary's husband informs her of his intention to divorce her, although he will still financially look after her and their child. Her reaction makes the reader feel sympathy for her stunned disbelief as she whispers, "I'll get the supper." 
Mary whacks her husband with a frozen leg of lamb. Mary had been so docile and sympathetic up to this point, and the gentle temperament of lambs is meant to underscore the humorous nature of Patrick's death.
Mary's quick calculation about how to pull off the murder surprises many readers. The sweet and docile housewife coolly concocts a convincing cover for the murder and outwits the very men trained to spot lies.  
The detectives are easily persuaded to consume the murder weapon/ evidence. When one detective urges the others to finish the lamb, he says they'll "be doing her a favor" as they dig in and speculate that the murder weapon must be close by, "probably right under our very noses." 
Dahl is able to engage readers by subverting their expectations of where the story will go and how the characters will behave.