To consider Mary Maloney's mental state, consider her actions and thoughts before her husband delivers his bombshell:

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 in 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.

These are not the reflections of an insane woman. This is a devoted wife who carefully plans her quiet evenings with her husband. She respects his need to sit quietly and knows his favorite drink. She simply longs to sit in his presence after being apart all day and finds this time "blissful."

The news her husband delivers simply shocks Mary beyond measure. She cannot fathom that a man whom she adores and whose baby she is carrying could possibly be planning to leave her. As she tries to process this sudden change, her husband delivers one final insult and Mary reacts almost reflexively:

All right then, they would have lamb for supper. She carried it upstairs, holding the thin bone-end of it with both her hands, and as she went through the living-room, she saw him standing over by the window with his back to her, and she stopped.

“For God’s sake,” he said, hearing her, but not turning round. “Don’t make supper for me. I’m going out.”

At that point, Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.

It is important to note that Mary didn't pause in this series of actions. This wasn't a premeditated action. Further, it is clear that Mary reacted in shock by the clarity that returns as her husband's body hits the ground:

The violence of the crash, the noise, the small table overturning, helped bring her out of he shock. She came out slowly, feeling cold and surprised, and she stood for a while blinking at the body, still holding the ridiculous piece of meat tight with both hands.

When Mary realizes what she's done, she becomes quite calculating in her next steps. Fearing that she will suffer death for her murderous reaction, Mary decides to take immediate steps to save not herself but her unborn baby. She is willing to do whatever it takes to make sure she is not killed before her baby is born.

Mary devises a thorough plan to cover her actions, generating a solid alibi with the grocer, and then feeds the murder weapon to the detectives who come to investigate.

Mary's actions are not those of an insane woman but of a calculating woman who effectively covers her crime in order to save her unborn child. Though her actions certainly don't prove her a virtuous woman, they do prove that she is thinking quickly and effectively in the aftermath of her crime.

Admittedly, Mary kills her husband with a frozen lamb chop in a fit of rage, which, on the surface may appear an insane act. However, Mary knows exactly what she was doing, and the crime makes logical sense.

Mary is very angry and knows her life will be ruined if her husband divorces her, as he says he will. She has invested everything in her relationship with him, catered to him, and is heavily pregnant with his child. From her point of view, her own life—in terms of what seems a life worth living—is seriously threatened by his proposed action. He is planning to strip her of her social status as a wife and leave her a single mother coping with a baby on her own.

While none of this is an excuse for murdering another person, the act makes rational sense given Mary's life context. She also probably finds a bit of poetic justice in killing him with the very food she bought to nurture him.

Mary was a sane woman except for the momentary lapse in judgment which she suffered due to anger. She was angry towards her husband who had made it clear that he wanted a divorce. The husband offered her no opportunity to talk about it or even ask questions regarding the reasons for the divorce. Mary attempted to ignore the situation and proceeded to inform her husband that she was going to prepare supper. Her reactions were perfectly normal given the weight of the situation. She was most likely going through shock and at the same time trying to process the event. The husband may have taken it too far by refusing to eat, leading her into a fit of rage that culminated in the murder. Her pregnancy made matters worse because her husband’s rejection was not only being directed at her but to their unborn child.

To further confirm that she was sane, Mary was able to devise a favorable sequence of events leading to her husband’s death. She secured her alibi by going to do some groceries and destroyed the murder weapon before the police came. The murder weapon was a frozen leg of lamb, which she cooked and ironically served to the policemen, who came by her house to conduct the preliminary investigations.

Arguments against Mary’s sanity

Depending on how one looks at it, Mary may also have displayed some psychopathic characteristics in the form of:

• Leading a parasitic lifestyle by being seemingly clingy to her husband
• Failing to accept responsibility for her own actions and instead choosing to avoid arrest
• Exhibiting poor behavioral control by bludgeoning her husband to death with a frozen leg of lamb
• Showing her lack of guilt by chuckling when one of the officers suggests that the weapon could be right under their very noses