The main idea in the story is informed by irony. The story suggests that what we see is not what we get and that we are easily mislead by our perceptions of what is supposedly good and wholesome. That is the deception. Roald Dahl plays on our preconceived notions by informing us through Mary, her husband and the other characters, how fragile such beliefs can be. We are shocked and surprised by the unfolding events and Mary's reaction at the end of the story. Furthermore, our own values are tested. Did Patrick Maloney get his just desserts? Has justice been served or is Mary Maloney nothing more than a common criminal - a sly and ruthless murderer?

Whatever we decide - the message is clear: Good can be bad and bad can be good. Mary Maloney is presented as a loving, docile, caring wife who would do everything in the world to please her husband. She tends to his every whim and slavishly serves him. Her physical description equates her to a lamb - harmless and obedient, loyal to its master/sheperd. She does, however, display her darkest side when she (almost unconsciously) lashes out and kills her husband. She expresses no grief and calmly goes about the business of creating a perfect alibi.

The ending suggests a lack of remorse. Mary's giggling informs us that she takes an almost psychotic pleasure in having successfully allowed the policemen to obliterate the evidence. She was innately bad all along and her husband's rejection triggered her evil. Mary presented a false image of goodness. Mary did a bad thing, but her actions suggest that, for her, it was good.

Furthermore, Patrick Maloney is a detective and an officer of the law and one tends to believe that such individuals are, or should be, exemplary in terms of their conduct. He, however, seems to have been involved in an illicit affair and chooses to leave a loving, doting partner to further indulge his immorality. Once again, we are surprised that such a one, a protector, is willing to abandon his vulnerable and pregnant wife. He is not regretful and remorselessly, clinically, spells out his plans to his ever-attentive spouse.

It is also surprising how gullible and naive the investigating officers are, and they are responsible for our security and the protection of our rights. They are fixated on finding proof and are easily deceived by the sly Mary and, just as we are generally mislead by those who present a good image, are they caught up in this naive assumption that she could not be responsible for such a terrible deed. They go through the motions and do what is expected and we can be confident that that would probably be the end of it all. The post-mortem investigation would indicate that officer Maloney was the victim of blunt force trauma but how and why would the good Mrs Maloney ever commit such a heinous crime? It had to be someone else.

In the final analysis then, the appearance is not always the reality. 

The main idea of "Lamb to the Slaughter" is one of murder and deception. Although the marital problems and difficulties play a huge part in the main event, the actual main event/idea is the murder and subsequent deception that take place in its aftermath.

The story is a brief portrait of domestic violence and betrayal. I'm not sure if there is one main idea beyond writing an entertaining story with a bit of a twist ending, but one might say that the central theme is betrayal. First the husband betrays the wife by announcing his plan to leave, then the wife betrays his basic human trust by murdering him. Other themes are identity, especially shown in the way the wife has internalized the identity of a 'normal', middle class wife.