Mary is completely in love with her husband. She is infatuated with every aspect of him.
Mary enters a completely emotionless state and kills her husband.
Mary went to the store in the midst of trying to cover up the murder. She went to get food and was going to use that as her alibi.
When Mary got home from the store and saw her husbands dead Boyd laying on the floor, she had realized what she had done and was completely heartbroken.
Mary had cooked the leg of lamb she used to kill her husband and served it to the detectives. They were talking and the detectives came to the conclusion that the murder weapon wouldn't be hard to find. This made Mary giggle.