Explain how Mary Maloney, Patrick, the detective, the unborn child, the leg of the lamb could be the 'lamb to the slaughter.'

The title of the story suggests that there is more than one meaning implied. It could be a double-entendre, or a triple-entendre (if there is such a thing), or a quadruple-entendre. Patrick is a big, tough cop, but he is the most obvious "lamb" to the slaughter because he actually is slaughtered. And he is slaughtered like an innocent lamb because he has no idea what is going to happen to him. He is innocent in another sense. He doesn't understand his wife at all--so he doesn't suspect that it is a bad idea to have his back turned to her, especially at that juncture.

Mary is very much like a lamb, at least up until the time that she succumbs to a fit of rage and bashes her unsuspecting husband over the head with the frozen leg of lamb. She is meek, mild, innocent, trusting, passive, and totally naive. It almost seems as if her whole world is her little house, the neighborhood grocery story, and the restaurant to which her husband takes her once a week. She trusts in Patrick like a little lamb--until he tells her what is on his mind.

The frozen leg of lamb, which is the centerpiece of the story, could be "the lamb to the slaughter" because it actually is a lamb and because it actually goes to the slaughter of Patrick Maloney. Without the leg of lamb there would probably have been no slaughter. Mary would not have gone to seek a weapon. She only committed the deed because she happened to have the perfect weapon in her hand.

The unborn child is like a lamb because, like a lamb, it is a baby. It is curled up peacefully inside its mother's womb. It is completely innocent. Nevertheless, it plays an important part in the "slaughter" of its father, for if Mary were not six months pregnant, and if she were not looking forward to motherhood and a happy domestic life, she probably would not have reacted to her husband's rejection with such savage rage. 

So Mary, Patrick, the unborn baby, and the frozen leg of lamb are not just "likely suspects" but are all simultaneously involved in the "slaughter" of Patrick Maloney. The choice of the title is brilliant. It is also comical, which is characteristic of Roald Dahl. Being funny, the title suggests that the whole story is not to be taken too seriously. The reader can't help being amused by the picture of a woman killing her husband with a frozen leg of lamb, and the reader can't help being amused by the final scene in which Mary gets the policemen to devour the incriminating evidence.