Before committing the murder, Mary behaves in the same way that she would on any other evening. She prepares the home for her husband's arrival by lighting the lamps and then preparing the glasses for their drinks. When Patrick arrives at the house, she removes his coat and fixes them both a drink.
After the murder, Mary washes her hands, "fixes" her make-up and puts the lamb in the oven before going to the grocery store. There, she buys some vegetables before coming home and reporting her husband's death to the police. When the police arrive, she answers some questions while they look around for the murder weapon. In the middle of the search, she feeds them the leg of lamb she used to murder Patrick.
What is significant about Mary's behavior before and after the murder is that she never deviates from her domestic role. Before the murder, she focuses on the well-being of her husband. After murdering him, she continues as normal, buying items from the store and then feeding the police officers at her home. It is, perhaps, this continuity in her behavior that allows her to get away with her crime.