What is the irony behind the title of "Lamb to the Slaughter" by Roald Dahl?

The term "lamb to the slaughter" is usually used figuratively rather than literally. As a figure of speech, it means an innocent person who is being set up as a sacrifice or dupe without realizing what is going on.
At first, it seems as if the innocent, sixth-month pregnant Mary—who looks like the Virgin Mary with her translucent skin and big eyes—is the lamb to the slaughter: we find out that she is being sacrificed to her husband's desire for a divorce, presumably so he can marry another woman. She seems to have done everything to be a good wife. She seems to be an innocent victim of her husband's selfishness.
Situational irony is when the opposite happens from what is expected in a story. Verbal irony occurs when words mean the opposite of what a reader expects. In the phrase "lamb to the slaughter," Dahl uses both situational and verbal irony. Mary, unexpectedly, is not the lamb to the slaughter: the situation works out in the opposite way. Sweet Mary slaughters her husband. And in a use of verbal irony, she literally uses a lamb (leg of lamb) to slaughter him. The phrase "lamb to the slaughter" is literal, not figurative, because she whacks her husband on the head with a frozen leg of lamb and kills him.

The irony of the title lies in its ambivalent implications; through his fantastic story, Roald Dahl introduces an unlikely twist to the way this famous phrase is interpreted. He chooses to focus on a more direct interpretation rather than emphasize the conventional metaphor we are used to.
The phrase "lamb to the slaughter" is typically an allusion to Christ being led as an innocent to his undeserved death. In this story, the narrator chooses to use the phrase to characterize an actual leg of lamb as a weapon. The leg of lamb is literally the instrument of slaughter.
In the story, Mary Maloney is a faithful and loving wife; she is a woman who enjoys waiting on her husband. One evening, her husband comes home visibly agitated, only to confess that he has decided to leave her. The news shocks Mary to the point that she finds it difficult to function. At six months pregnant, Mary is in an unenviable position.
After her husband's refusal to eat dinner and his injunction that she not make a fuss, Mary whacks him on the back of his head with a frozen leg of lamb.