Mary was calm and complacent during her marriage. She kept a neat house, and waited happily each day for her husband to come home from work. In this way she was much like a lamb.
When her husband calmly tells her it is over, an inner rage causes her to slaughter her husband. She feels little in the way of remorse, as can be seen by her quick planning to cover up her crime.
In some ways , it can be seen that her husband was the lamb. He believed Mary would complacently accept his decision to leave, without a fuss. He was not threatened in any way for her to react with violence. He was killed easily, like leading lambs to the slaughter.
My class came up with how in the story, parts talked about Mary's "soft skin" and "dark eyes" like a lamb. She eventually lays down the slaughter. The title is ironic because usually you think of slaughter to the lamb, then someone eats the lamb. But, the lamb brought the slaughter, and someone eats the lamb. Also, my classmate thought this story was an allusion to "Mary had a Little Lamb". When she said that, we all shuddered. By the way, if you liked this story, read The Way Up to Heaven, Roald Dahl. I'll just say that they're similar in plot twists.
Lamb to the slaughter,
i think that the title has somekind of ironic sense refering to Ms. Maloney. Lamb is supposed to be pure, innocent like Mrs. Maloney who is a docile, loving, subservient housewife however things has changed, and lamb was used as a killing weapon and a manipulative tool (slaughter). In other hand, Mrs. Maloney turns into an independent, cruel woman.