Explain why Mary's behavior in the store is so important
Mary's behavior in the story is important because of how much and how quickly it changes. When the story begins, Mary is the quintessential doting wife.
Now and again she glanced at the clock, but without anxiety: She merely wanted to satisfy herself that each minute that went by made it nearer the time when he would come home. . . When the clock said ten minutes to five, she began to listen, and a few moments later, punctually as always, she heard the car tires on the stones outside, the car door closing, footsteps passing the window, the key turning in the lock. She stood up and went forward to kiss him as he entered.
Whoa. That's some serious love (infatuation?) to her husband. She is literally counting down the minutes until he comes home. She won't even let the man get himself a drink. She runs to do it for him.
Of course Patrick doesn't seem to care about any of this. He quickly tells his wife that he believes their marriage should no longer continue. Mary is dumbstruck. She's like a walking zombie for the next few minutes. She goes down to get the leg of lamb and decides to cook him dinner anyway. Patrick reiterates that he doesn't want dinner, and Mary calmly kills him.
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. She might as well have hit him with a steel bar.
Then Mary turns into a very cold and calculating woman. She concocts an alibi and proceeds to establish that alibi. She also cooks the leg of lamb in order to destroy (eat) the murder weapon.
All right, she told herself. So I've killed him. It was extraordinary, now, how clear her mind became all of a sudden. She began thinking very fast. . . She carried the meat into the kitchen, put it into a pan, turned on the oven, and put the pan inside. Then she washed her hands, ran upstairs, sat down in front of the mirror, fixed her makeup, and tried to smile.
Her calm behavior is a critical insight into her character. It shows a lack a remorse on her part. She may have been a doting wife, but she pulls about the quickest 180 in history once she realizes that Patrick doesn't reciprocate her feelings. Once she realizes that, everything about her actions indicate that she is looking out for herself and her soon to be born child. I find it quite easy to sympathize with her on one hand.
On the other hand, what about the baby? What were the laws about murderers with unborn children? Did they kill them both -- mother and child? Did they wait until the baby was born? What did they do? Mary Maloney didn't know and she wasn't prepared to take a chance.
On the other hand, I wouldn't want to cross her and be led like a lamb to the slaughter.