The three key issues encapsulated within the story are justice, stereotyping and passion.
We should be repelled by Mary's actions in killing her husband and then working to a carefully calculated plan to cover up her crime. However, we are impressed with her ingenuity in cooking the leg of lamb and convincing the policemen to eat it. We have empathy for her as a loyal, pregnant wife who was to be quietly cast aside by her husband. Whether itis justice that he pays for his insensitivity is an issue for the reader to debate.
Mary Maloney is not suspected of the crime as she is the stereotypical loyal wife. She is waiting for his return, ready to pour Patrick's whisky and see to his meal. She is happily carrying his child. The dramatic irony in the police looking for the heavy blunt instrument as they prepare to eat the weapon relies on tehm not suspecting the gentle loving wife.
Mary Maloney has a passion for her husband and their happy existence. This changes at the news he is to leave her and her passion is transferred to the anger of Patrick's deception, then defending her unborn child. Mary outwardly exudes calm propriety, but is actually a driven woman.