Did Mary implicate herself in the crime at any point in "Lamb to the Slaughter"? Did she leave any clues that might have led to her arrest?
This is a carefully plotted story. Mary Maloney had to have a motive for killing her husband, but it had to be a motive no one would have suspected. It was essential that she be above suspicion. This would seem necessary in any perfect-crime story in which the murderer gets away with it. That is why Roald Dahl refrained from giving the husband any obvious faults.
Patrick was not a heavy drinker. Mary notices that he is drinking rather heavily that evening.
He lifted his glass and drained it in one swallow although there was still half of it, at least half of it left.
When he came back, she noticed that the new drink was dark amber with the quantity of whisky in it.
These observations characterize Patrick as a strong, silent type who is finding it hard to say what he has to say. They also prove that he is not normally a heavy drinker. Patrick cannot have told anybody about his wantinig a divorce, since he is finding it so hard even to tell his wife.
From the way Patrick speaks to Mary, it is obvious that he is not an abusive husband. He is cold, but he does not threaten or insult her. He does not appear to be a womanizer, either. He comes home right around five each workday. He tells her:
"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."
Patrick has not done or said anything away from home which would give anyone cause to suspect that he does not have a perfect marital relationship with Mary. This means that she has the most important element of a perfect murder working in her favor. If the police thought she might have had a motive for killing her husband, they might have tried much harder to find any evidence against her.
It was a good idea to make Patrick a policeman. He could have made a lot of enemies in his work. That plus the brutality of the murder detracts from any possible suspicion that Mary could have been responsible. It looks like a crime that could only have been committed by a strong man using a metal club--and it only looks that way because Mary was using a frozen leg of lamb and acting in a spontaneous fit of rage.
The fact that the victim was a policeman works in Mary's favor another way. It brings a horde of fellow policemen to the house. There are far more men investigating the crime than usual because he was "one of ours," and they stay longer than detectives and uniformed cops would normally stay at a crime scene. This does two things: It gives Mary plenty of time to cook the leg of lamb, and the men work long past their regular dinnertime, so that they are hungry enough to consume the whole murder weapon.
Once the leg of lamb has been consumed, it is hard to see how the police could find any evidence against Mary--especially since she is known to be such a meek, devoted wife. She is not pretending to be loving and devoted; she actually is that way--up until the time when her husband makes the totally surprising and devastating announcement that he wants a divorce.
A spouse is almost always the first person to be suspected in a murder case. This is because the spouse is usually the one who is guilty. But Mary Maloney gets away with the perfect crime because she is above suspicion and because her husband preserved such a clean reputation that no one would suspect he could have given his wife any cause to want to kill him.
There is no motive and no murder weapon. It must have been an outside job!