Theme of Appearance and Reality in Roald Dahl's Lamb to the Slaughter

In the story it appears that Mary is the weaker character. She waits patiently at home for her husband to return, and waits on him like a servant. She is pregnant, and we see this as making her vulnerable too. When Patrick tells her he is leaving her, we see her as the victim of the tale; even more so when he reveals that he does not want the breakdown in the relationship to tarnish his career-
"Of course, I’ll give you money and see you’re looked after. But there needn’t really be any fuss.’’
Patrick is selfish and inert to the maelstrom of emotions his news creates in the mind of his devoted, pregnant wife. Once she has killed him opportunistically with the leg of lamb, we are impressed at her ability to focus to set up her alibi. As she has been the devoted wife, there is no reason for anyone to think otherwise of Mary. The grocer is happy to help her organise the missing ingredients of her husband's supper, and the police from Patrick's station are sensitive to the shocked and grieving pregnant widow. She is in shock, but more about how she will manage to conceal the knowledge of her crime to preserve herself and her unborn child.
The serving of the roasted lamb fits with Mary's persona as a devoted and benevolet housewife, who would of course be indebted to her husband's colleagues for their prompt and sensitive investigation in to the crime. Mary has always been methodical in her domestic duties and simply appears to be continuing to do so, when in reality she is destroying the evidence of her crime.