she initially doesn't care what happens to her because she feels justified in sacrificing Patrick. At this point, the baby is in danger of being sacrificed (killed or given up). Mary thinks: 

On the other hand, what about the baby? What were the laws about murderers with unborn children? Did they kill them both -- mother and child? Did they wait until the baby was born? What did they do? Mary Maloney didn't know and she wasn't prepared to take a chance. 

Mary didn't want to sacrifice the baby, so she devised a scheme to make sure she eliminated herself as a suspect. (One could make the case that the baby was sacrificed in the end anyway, in having to live with a homicidal mother, but that is a speculative assumption.) 

One could say that Mary giving them the lamb was the same as a person offering up a lamb or some other symbolic object to God; in return the person (Mary, in this case) would get something: elimination as a suspect as the detectives unknowingly eat the evidence. However, this still makes the lamb the sacrifice.

To call the detectives themselves as sacrifices, one would have to say that they were given up by someone to someone or something, for some purpose. One could say that Mary sacrificed them by strategically having them eat the evidence. Thus, they became the last sacrifices because in eating the murder weapon, they unknowingly eliminate the evidence, and in the end they will have to give up (sacrifice) the case. 

P.S. The detectives may all be considered "lambs to the slaughter" because they are brought to the Maloney home by the slaughter of Patrick, and because, like innocent lambs, they are so completely fooled by Mary's alibi and her pretense of grief that they never suspect her of being responsible for her husband's death. The author has set the story up in such a way that there can be no suspicion of Mary having any motive. Patrick is a very conscientious cop. He doesn't want the breath of a scandal. His conclusion to his long speech is intended to show this.

"So there it is," he added. "And I know it's kind of a bad time to be telling you, but there simply wasn't any other way. Of course I'll give you money and see you're looked after. But there needn't really be any fuss. I hope not anyway. It wouldn't be very good for my job."

There is also the fact that he keeps regular hours. Mary knows exactly when to expect him to come home.

Now and again she would glance up at the clock, but without anxiety, merely to please herself with the thought that each minute gone by made it nearer the time when he would come.... When the clock said ten minutes to five, she began to listen, and a few moments later, punctually as always, she heard the tires on the gravel outside, and the car door slamming, the footsteps passing the window, the key turning in the lock. 

The above paragraph is solely intended to show that Patrick is leading a regular home-life. Mary always knows where her husband is--either at home or at work. 

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.