Enotes has a wonderful summary of this story. I have included the link. In brief, however, this is a story about a woman who is the symbol of a contented middle class housewife. Her world is blown apart when her husband announces that he is going to leave her. She uses a frozen leg of lamb to kill him and then plans a clever alibi to cover it up. The "lamb" is a symbol of innocence, and readers are shocked that this seemingly innocent woman can behave so violently. She has been slaughtered by her husband's betrayal, but she is the slaughterer as well, betraying her appearance of innocence.