Could you give an explanation for the title of the story "Lamb to the Slaughter" by Roald Dahl?

The more times that I am forced to consider the title of this story, the more I think it is the perfect title.  
First, Mary is initially presented to the reader as a lamb.  A lamb is typically docile, meek, and mild.  Mary is waiting in the front room for Patrick to come home.  Once Patrick is home, she attends to his every need and then quietly sits near him.  She is perfectly content to bask in his glorious manliness.  

She loved to luxuriate in the presence of this man, and to feel – almost as a sunbather feels the sun – that warm male glow that came out of him to her when they were alone together. 

She is the lamb and Patrick is the protector.  That is until Patrick decides to metaphorically slaughter Mary.  He tells her that he is leaving her, and she is left completely devastated and broken. She is a slaughtered lamb.  
Mary goes to get dinner started in a daze.  She's practically a walking zombie.  Then Patrick announces that he is going out to dinner.  That's when the title of the story shifts meanings.  Mary is now the lamb coming to do the slaughtering. She is not the lamb to be slaughtered anymore.  She is the lamb to the slaughter of Patrick.  What's even better is that she brings an actual lamb to the slaughter.  Her murder weapon of choice is a leg of lamb.  

At that point, Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.