How does the author use elements of surprise, foreshadowing, and irony in Lamb to the Slaughter?

I will start with surprise. The story is loaded with surprises for the reader.  When the reader is introduced to Mary, she is the quintessential doting wife.  She is sitting at home eagerly awaiting the arrival of her husband.  When Patrick finally does get home, she rushes to welcome him, calls him "darling," and is eager to get him a drink and have him sit and relax.  The reader can only assume that Patrick's feelings...

I will start with surprise. The story is loaded with surprises for the reader.  When the reader is introduced to Mary, she is the quintessential doting wife.  She is sitting at home eagerly awaiting the arrival of her husband.  When Patrick finally does get home, she rushes to welcome him, calls him "darling," and is eager to get him a drink and have him sit and relax.  The reader can only assume that Patrick's feelings for Mary are similar.  That's why it comes as quite a surprise that he tells Mary that he is leaving her or divorcing her.  It's also surprising to see that Mary, despite her meek introduction, kills Patrick with a single blow.  Mary keeps the surprises coming, because she doesn't panic and fall apart at her murderous deed.  Rather she collects herself and goes about setting an alibi in order to get away with the murder.  

Dahl foreshadows the murder and the murder weapon when he has Mary specifically mention the lamb in the freezer.  

"We can have lamb. Anything you want. Everything's in the freezer."

Mary is absolutely correct.  Everything she needed was in the freezer.  

There is irony in the title of the story.  Most readers likely assume that a "lamb to the slaughter" literally means a lamb being killed.  As a metaphor, the phrase is often used to describe a person being killed.  Jesus is often referred to in this manner.  But in this story, the lamb that is coming to the slaughter is a lamb that is actually doing the slaughtering.  The lamb kills Patrick.  It's totally unexpected.  

There is dramatic irony at the end of the story as well.  The reader knows what has happened, and knows that the murder weapon is literally right under the noses of the police investigators.  

"It's probably right under our noses. What do you think, Jack?"

That's because they are eating the murder weapon. Yum.