The title "Lamb to the Slaughter" is a double entendre. The obvious reference is to the frozen leg of lamb which Mary Maloney uses to kill her husband. The title is actually very clever, since the other meaning of the title could be taken to refer to the husband who is totally unsuspecting and seems to fit the biblical allusions:
He was oppressed and He was afflicted, yet He opened not His mouth; He was led as a lamb to the slaughter, and as a sheep before its shearers is silent, so He opened not His mouth. ISAIAH 53:7
The place in the Scripture which he read was this: "He was led as a sheep to the slaughter; and as a lamb before its shearer is silent, so He opened not His mouth." ACTS 8:32
Since it was Patrick Maloney who was "slaughtered," a strong case could be made that the title also refers to him. Mary, however, is the viewpoint character and gets far more attention in the story from beginning to end. At first she is portrayed as an extremely meek and gentle woman, very much like a lamb.
Her skin--for this was her sixth month with child--had acquired a wonderful translucent quality, the mouth was soft, and the eyes, with their new placid look, seemed larger, darker than before.
... and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house.
She is portrayed as exceptionally gentle right up to the point where she suddenly releases her pent-up anger at the man who is deserting when she is six months pregnant and after her years of slavish devotion.
Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.
She might just as well have hit him with a steel club.
So the second meaning in the double entendrecould apply either to Patrick or Mary Maloney. It seems more likely that Roald Dahl intended it to apply to Mary, since she is the major character, the protagonist, and the viewpoint character; whereas he husband has only a minor role and is dispatched early in the story.