In "Lamb to the Slaughter," how does Mary's behavior in the grocery shop contradict what has happened earlier in the story? Why is her visit to the grocery shop an important piece of the story?
Before Mary heads to the grocery shop, she kills her husband by hitting him in the head with a frozen leg of lamb. She then practices her "speech" to the grocer and proceeds to create a ruse that will exonerate her from any murder charges.
Once Mary kills her husband, she goes upstairs and rehearses what she will say to Sam, the grocer. It's not clear what she's planning as she contemplates her possible jail time. However, when she gets to the grocer, we see the behavior of someone who has faced no trauma or tragedy. Mary acts as if nothing ever happened. She orders some food, chats with Sam, and takes the food back to the house.
As she's walking back, the narrator reinforces her new storyline, telling the reader, "she must cook it good, and make it as tasty as possible because the poor man was tired." She is creating a new narrative that completely contradicts the fact that she just killed her husband. Her husband is not tired; he's dead! She is rewriting herself out of the narrative.
When she returns to the house, she "discovers" her husband on the floor, presumably dead, and reacts as such. She calls the police and tells them her new story of grabbing some extra food for dinner and returning home to find him injured on the floor. Now, back to the grocery shop.
Mary's trip to the grocery shop is important for a few reasons. First, it gives her a possible alibi in that someone now knows where she was during the time of the murder. Sam can attest to the fact that Mary was her "normal" self and picked up some of her husband's favorite foods. The trip also helps her push the food onto her guests, which in turn destroys the murder weapon and any evidence of her guilt. She has this large meal prepared that is reminding her of her newly deceased husband, and she guilts the cops into eating all the evidence. The trip to the grocery shop, while it contradicts the reality of her behaviors, becomes the linchpin in her manipulative plan.