"Lamb to the Slaughter" is a perfect-crime story. The author has to establish certain facts which make it possible for Mary Maloney to commit that perfect crime successfully. The fact that she kills her husband with a frozen leg of lamb and then manages to dispose of the murder weapon by having the investigators eat the murder weapon is not sufficient in itself. Dahl establishes that Mary is an exceptionally loving, devoted wife. This is one factor that keeps the investigators from questioning her too thoroughly or delving too deeply into her alibi. She is above suspicion. Most of the investigators have known Patrick Maloney personally for years. They have never heard any hint either that he was having an extramarital affair or that there was any friction between him and his wife. When Patrick tells Mary that he hopes there will be no "fuss" because "It wouldn't be very good for my job," this is Dahl's way of informing the reader that Patrick has been careful to avoid letting any of his colleagues know that he is becoming tired of his wife or that he is thinking of getting a divorce. If the investigating officers had any clues about domestic problems between the victim and his wife, they would focus more attention on her. She manages to get by with the perfect crime because everyone believes they are a completely happily married couple. And the fact that she is expecting a baby would only seem to make them bonded more tightly. The fact that she is six months pregnant would also make it seem quite unlikely that she would be capable of committing such a brutal murder or that she would want to murder the man she was going to be dependent on for financial support. There are many details in the story that contribute to Mary's success in committing the perfect crime. Another one is the fact that she is married to a cop and knows how cops think, while the fact that Patrick is a cop makes it seem plausible that he could have been murdered by someone he had been responsible for sending to prison.

There is a void in the relationship as the story opens.  From all accounts, Mary is a dutiful wife.  She tends to her husband's needs and does not display any signs that she is a bad wife or someone who is unresponsive to her husband.  Mary sits silently while he tells her that he is leaving her. The true intent of her husband's words can be seen in the follow up line to the breakup:

"So there it is," he added. "And I know it's kind of a bad time to be telling you, bet there simply wasn't any other way. Of course I'll give you money and see you're looked after. But there needn't really be any fuss. I hope not anyway. It wouldn't be very good for my job."

The emotionally blunt manner in which he speaks to her indicates much about the level of fault in their relationship.  Certainly, she is not justified in beating him to death with a leg of lamb.  Yet, he does not really display much in way of sensitivity to her as he ends their marriage.  He speaks of how he hopes there would not be "any fuss."  He reflects on how this "wouldn't be very good for my job."  Even when she leaves to cook the fateful dinner, he yells that he is not going to be staying in that evening.  It is evident that he wants to leave this relationship at all costs, leaving her in a condition of loneliness.  It is hard to blame him for the problems in the relationship because Mary kills him with the leg of lamb.  Yet, I think that Mr. Maloney's attitude towards both the marriage and his desire to get out of it reflects how he might be more to blame for what she does to him.