The situational irony at the heart of “Lamb to the Slaughter” is the fact that Mary, a docile housewife who loves her husband Patrick deeply, brutally kills him in a fit of rage. Readers do not see this coming, especially after passages like the following from the beginning of the story:
[Mary] loved [Patrick] for the way he sat loosely in a chair, for the way he came in a door, or moved slowly across the room with long strides. She loved the intent, far look in his eyes when they rested on her, the funny shape of the mouth, and especially the way he remained silent about his tiredness, sitting still with himself until the whiskey had taken some of it away.
This passage clearly communicates how loving Mary is and how much her life centers around Patrick. Even after years of marriage, she loves the simple and mundane things that Patrick does, such as “[sitting] loosely in a chair” and “mov[ing] slowly across the room with long strides.” She also loves “the funny shape” of his mouth and how quiet he is when he comes home from work. None of these descriptions point to underlying resentment or violent impulses on Mary’s part.
The ironic twist of Mary killing Patrick in a fit of rage after he announces he is leaving her conveys to readers that a large betrayal can trigger violent impulses in anyone, and that viewing women as only loving actually reinforces sexist gender expectations.