In Lamb to the Slaughter, what causes the incident when Patrick tells her he is leaving her?
The author, Roald Dahl, does not explain why Patrick is leaving Mary, nor does Dahl record the explanation Patrick gives her when he tells her he wants a divorce. The reader is expected to deduce Patrick's motivation as well as Mary's from the representation of their characters and from what they actually say to each other.
“This is going to be a bit of a shock to you, I’m afraid,”, he said….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.
Patrick is the strong, silent type of man who finds it hard to talk about his feelings. But he has made a hard decision and it is obviously irrevocable. The reader will probably imagine why he wants out of the marriage. His wife is a clinging, needy, dependent, overly attentive woman who is suffocating him with all her care and attention. Her dialogue is full of such things as:
“I’ll get it!”
“Darling, shall I get your slippers?”
Without having any explanation of their motives, the reader can still understand why Patrick should be tired of this woman and should also be able to understand why she reacts with a sudden outburst of fury. She has done everything she can to keep his love. She has been his devoted slave. She has been a "doormat"--and now she is six months pregnant.
If Mary hadn’t happened to be holding a frozen leg of lamb when Patrick said, “I’m going out,” the murder might never have taken place. Mary acted without thinking.
Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.
She might just as well have hit him with a steel club.
Dahl was clever in choosing to make his main characters a cop and a cop's wife. Mary knows a lot about how the police work and how they think. She displays a remarkable change of character after killing her husband with the leg of lamb. She must have been acting a lot more dependent and submissive than she really felt, thinking erroneously that this was how to hold her husband. She becomes quite quick-witted and resourceful when she realizes she needs to save herself from prison and save her unborn child from an orphanage. She establishes an alibi before calling the police and reporting that an intrudeer has murdered her poor husband. In the meantime the leg of lamb is cooking in the oven at the highest temperature.
Because her husband was "one of their own," the police devote extraordinary attention to the case. The Maloney home is swarming with uniformed and plainclothes policemen. They spend many hours searching for the murder weapon, giving the leg of lamb plenty of time to cook and giving themselves plenty of time to work up appetites.
No one suspects Mary because she is known to be an exceptionally loving, devoted wife. There is no indication of another woman in Patrick's life. Dahl takes care to eliminate that from the husband's motivation by establishing early in the narrative that Patrick always came home at the same time and stayed home with Mary. So the police naturally assume that some enemy Patrick made as a cop broke in and killed him while his wife was at the grocery store. They can never find the murder weapon because the hungry cops have devoured it.