Why does Mary force the police officers to eat the lamb?

Dahl's "Lamb to the Slaughter" is one of the best examples of dramatic irony. At the very end of the story, after Mary has killed her husband with a frozen leg of lamb and the police have come to investigate, Mary ends up feeding the cooked lamb to the policemen, thereby disposing of the murder weapon. While the reader is fully aware of the this, the police continue to enjoy the feast that Mary has provided them.
This act is done to complete the "perfect murder" scenario that Mary has upheld throughout the story. She is the only one who knows that she has a motive to kill her husband; she goes to the grocery store with calm ease; she speaks to the police in a way that communicates both shock and kindness. By ridding herself of the only thing that could point back to her being the murderer, Mary feels relief and almost a giddiness—she has gotten away with murdering her husband. In fact, as the police finish off the leg of lamb, the very last sentence of the short story is, "And in the other room, Mary Maloney began to laugh."

In "Lamb to the Slaughter," Mary forces the police officers to eat the leg of lamb because she does not want her guilt to be revealed to them.
Remember that Mary used the frozen leg of lamb to kill her husband. She hit him over the head with it after he said that he was leaving her. Although her crime was not premeditated, Mary still committed murder, meaning that she would go to prison for a long time if the officers find out what she did.
Instead of facing up to what she has done, Mary concocts a cunning plan. She proceeds to cook the leg of lamb and then offers it to the investigating officers who have stayed late at her house. By doing this, Mary portrays herself as being kind and caring, not a cold-hearted murderess.
This scene, therefore, acts as an example of dramatic irony since we, the reader, know that she is disposing of the murder weapon. The officers, however, think that the meal is just a token of her kindness.