It is clear from the text that Patrick Maloney has taken his wife, Mary, for granted. She has, throughout their marriage, been his loyal servant - doting on him and satisfying his every whim. She is described as a mild-mannered, placid individual - as gentle as a lamb.
There was a slow smiling air about her, and about everything she did. The drop of a head as she bent over her sewing was curiously tranquil. 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.
We learn that:
For her, this was always a blissful time of day. She knew he didn't want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house. She loved to luxuriate in the presence of this man, and to feel-almost as a sunbather feels the sun-that warm male glow that came out of him to her when they were alone together. She loved him 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.
Mary is clearly a loving, caring wife whose world literally revolves around her husband. Patrick believes that he can do whatever he pleases and that Mary would not challenge him. When he arrives home he receives the same kind of loving attention that he has become accustomed to, so there is no doubt in his mind that she would unquestioningly accept the devastating news he is about to give her. Mary is not even put off by his brusque manner and happily offers to do chores for him. It is quite easy for Patrick to accept that Mary does not impose any threat and when he tells her that he is going to leave her, pregnant and all, she:
sat very still through it all, watching him with a kind of dazed horror as he went further and further away from her with each word.
Patrick is deceived by Mary's calm, docile appearance. He has been treated so kindly throughout, why should anything be different?
The reality is, though, that Mary does have a different side to her. When Patrick rudely tells her:
"For God's sake, ... Don't make supper for me. I'm going out."
This is the final straw for Mary. In her brutal indignation, she swings the frozen leg of lamb, striking Patrick on the back of the head, killing him instantly. The mild-mannered Mary has, within a moment, turned into a murderer. Furthermore, she calmly creates an alibi and completes the meal with the frozen leg of lamb, the primary evidence, and later feeds it to the investigating officers. She has become the complete opposite of what we, her husband and his fellow officers believed her to be. The lamb has become a conniving, remorseless killer.