The title is justified since, firstly, in a direct and literal sense, the object used to kill Patrick Maloney is a frozen leg of lamb. Patrick is essentially "slaughtered" with a leg of lamb as the murder weapon. In this sense, then, the lamb, or rather, a part of it, is brought to the slaughter.
The title also has a metaphorical connotation. It generally refers to someone who goes innocently and unconcernedly into a dangerous or life threatening-situation. Patrick Maloney is a good example of this. Mary Maloney is described as someone who is meek and mild-mannered. She is a doting and loving wife who takes care of her husband's every need. She is, therefore, the last person anyone would suspect of committing a heinous crime, but that is precisely what she does. She kills the unsuspecting Patrick.
One can, furthermore, describe Mary as being "as gentle as a lamb." Ironically, it is this very same meek and mild-mannered person who, after killing her husband, coldly sets about creating an alibi to cover herself. She acts as if she is the traumatized victim of an offensive act when, indeed, she is the one who committed the deed. The fact that her gentle nature is not doubted adds credence to the title. Mary Maloney looks like the proverbial gentle lamb—but she is the one who committed the slaughter in this instance.
Furthermore, given of Dahl's physical description of her having calm, dark, large eyes and his explanation of her behavior toward Patrick, one would expect him to be good-natured and loving toward her. However, she becomes an unsuspecting victim of his selfish and uncaring attitude. He plans to leave her even though she is pregnant with his child. In this sense, then, Mary, who naively believes in her husband's loyalty and commitment, becomes an innocent victim of his ruthless attitude. She is essentially also, like an unsuspecting lamb, led on by Patrick until he divulges his heartbreaking intention. The news devastates her and she, without any forethought, lashes out. She also becomes, in the instant that she kills him, a victim of her own inner malice—a trait she most surely did not know she had until after she had killed him.
Finally, the unsuspecting detectives and other investigators, like lambs, innocently destroy the evidence by ingesting the murder weapon. They are unaware that the murderer is within their midst. Mary, to them, cannot possibly be held accountable for what happened to Patrick. She, however, takes morbid pleasure in the fact that, with each bite, the men are demolishing the only proof of her crime. She begins to laugh in the other room when she hears them talking about the evidence while they're actually eating it.
It is for all these reasons that the title aptly reflects the main and sub themes of Dahl's brilliant tale.