Mary insists upon this because the leg of mutton is what she used to kill her husband Patrick. Therefore the policemen will unknowingly destroy a vital piece of evidence that could have implicated her. The scene is a piece of consummate black comedy, of the kind that Dahl is famed for; a bunch of policemen sit and unknowingly eat a murder weapon.
The absurd humour of the situation is not lost on Mary as she listens to the policemen from another room, discussing the club that they think must have been used to kill Patrick:
'Personally, I think it's right here on the premises.'
'Probably right under our very noses. What do you think, Jack?'
And, in the other room, Mary Maloney began to giggle.
As in so many of his stories, for both adults and children, Dahl indulges in a bizarre and grotesque twist that completely upends a seemingly normal situation. Earlier, we have a serene domestic picture of Mary as a devoted wife waiting for her husband to come home from work. Then the sense of peace is shattered when he arrives only to tell her he’s leaving her, and she wastes no time in getting her revenge when she uses the frozen leg of lamb that they were going to have for dinner, to smash his head in. Therefore a seemingly innocent everyday item is transformed into something lethal; and Mary, a seemingly ordinary housewife, in an instant becomes a murderess, a fearsome avenger. Moreover, she manages to completely deceive the police.
Dahl also twists the meaning of a common phrase to use as the title of the story. The image of the lamb suggests innocence and purity; 'lamb to the slaughter’ normally means someone innocent who is about to be hurt in some way. The title could refer to Patrick, as he is the one who is killed, although he is not as innocent as the image of the lamb would suggest; he is planning to desert his wife, and tells her so in rather callous and abrupt manner.
The image of the lamb could also conceivably refer to Mary herself, as she appears very much a sweet, innocent type at first; but she commits slaughter, and in so doing, her own former innocence and goodness is sacrificed.
Finally, the title could also be taken in a literal sense, which plays upon the irony of a lamb (or a piece of it) actually becoming the instrument of slaughter for once.