The title is intended to make the story seem amusing, not to be taken too seriously even though it is a murder story. The title "Lamb to the Slaughter" has several different possible interpretations.
First, Mary Maloney uses a frozen leg of lamb to kill (slaughter) her husband Patrick?
Second, Patrick is caught completely off guard. This makes it easy for Mary to hit him over the head from behind. The way Patrick gets killed alludes to a passage in the Old Testament:
He was oppressed, and he was afflicted, yet he opened not his mouth: he is brought as a lamb to the slaughter, and as a sheep before her shearers is dumb, so he openeth not his mouth. Isaiah 53:7
And finally, Mary has always behaved as meekly as a lamb. It is a big surprise when she suddenly kills her husband in a moment of rage. She uses the weapon which happens to be in her hand, a frozen leg of lamb.
The whole story is based on the fact that the murder weapon is a frozen leg of lamb. The police look all over for a murder weapon but are unable to find one because Mary put the leg of lamb in the oven and turned the oven to the highest temperature. The investigating officers end up eating the evidence without realizing it. One of them says:
"Personally, I think it's right here on the premises."
"Probably right under our noses."