In "Lamb to the Slaughter" by Roald Dahl, what does Dahl want the reader to think/feel/understand about the story?

I think one thing that Dahl wants us to feel is the emotional shift that the reader goes through.  When the story begins, the reader can't help but love Mary Maloney.  She's pregnant and very much in love with Patrick.  She's the stereotypical doting wife.  Then to have Patrick so coldly dismiss her gives the reader even more reasons to love Mary and side with her, not Patrick.  But then she kills Patrick, which isn't that big of an emotional shift.  What gets me to strongly reconsider my opinions about Mary are her incredibly calm and calculated steps to get away with the murder.  In some ways, she is just as cold as Patrick.  

I also think about justice and injustice when I read this story.  Did Patrick receive justice?  What about Mary?  Surely not.  She got away with murder, but were her actions justified?  Was she even in her right mind when she hit Patrick?  I think those are all questions that Dahl wants readers to wrestle over.