Why does Mary go ahead and make supper for her husband after she has killed him?
At first, the reader thinks that perhaps Mary continues with the dinner plan out of shock at the murder she has just committed. Or perhaps she does so because she simply doesn't know what else to do. While her crime was one of passion--she killed her husband when he said he was going to leave her, in the middle of her pregnancy--she still cried genuine tears for his death, as she knows she loved him at one time.
As the story develops, it seems Mary has a pretty good idea what she is doing. She goes to the grocery and buys extras for the dinner, including dessert, so she might be cooking the dinner anyway so that she has an alibi for the crime. She also cooks dinner since dinner was the leg of lamb she hit her husband over the head with. That is, she is getting rid of the murder weapon in the perfect way. To add to the irony, the police detectives in her home eat the leg of lamb Mary has cooked, helping her to get rid of it.