just as if she is an actress preparing to go on stage. As she is coming home, she mentally runs through the "scene" ahead in her mind and determines what kind of emotions she is supposed to exhibit:

if she found anything unusual or terrible when she got home, then it would be a shock and she would have to react with grief and horror.

She tells herself to behave naturally, but she is scripting the whole evening to be an act. She pretends her husband is still alive as she comes through the door:

"Patrick!" she called. "How are you, darling?"

She decides it is easy to pretend. Her old "love" for her husband comes flooding back so that when she starts to cry, it seems to her that she is hardly acting at all. But she clearly is playing a part.

Mary has just killed her husband in a fit of rage, after he had obviously told her he was leaving her and wanted a divorce. She does not want to go to prison, especially since she is expecting a baby and knows the baby would be taken away from her. She has to establish an alibi. She goes to the grocery store in the hope of making it look as if some intruder had been waiting for an opportunity to sneak inside and murder her husband. Since he has been a policeman for many years, he could have made many enemies by arresting them and having them sent to prison. Mary needs time. She wants that leg of lamb to be thoroughly cooked so that it won't be recognized as having been used as a blunt instrument. This is a perfect-crime story in which the perpetrator escapes punishment because the murder weapon is disposed of in a unique way.

Mary knows the grocer well, since she shops there all the time. She engages Sam in conversation and explains why she is doing this last-minute shopping.

"I want some potatoes, please, Sam. Yes, and perhaps a can of beans, too. Patrick's decided he's tired and he doesn't want to eat out tonight," she told him. "We usually go out on Thursdays, you know, and now I don't have any vegetables in the house."

This type of personal relationship between a small shopkeeper and a customer used to be far more common in cities than in today's supermarkets. Sam will make an excellent witness, if necessary, to establish that Mary was grocery-shopping at the time her husband was killed. Meanwhile that leg of lamb must be thawing fairly quickly in the oven. The author establishes that Patrick always gets home at five o'clock and that it is nearly nine o'clock when Mary suggests that the searchers have a drink and must be close to nine-thirty when she asks them to eat the leg of lamb. The lamb has been cooking for about four hours, which seems just long enough for a frozen leg of lamb to get fully cooked.

Time is of the essence here. The author has to account for those four hours so that he can deliver his comical ending in which the policemen are gobbling up the evidence they have been seeking ever since they arrived. While they are eating every last bit of the murder weapon, one of them says:

"Personally, I think the weapon is somewhere near the house."

And another officer replies:

"It's probably right under our noses. What do you think, Jack?"

Before Mary heads to the grocery shop, she kills her husband by hitting him in the head with a frozen leg of lamb. She then practices her "speech" to the grocer and proceeds to create a ruse that will exonerate her from any murder charges.

Once Mary kills her husband, she goes upstairs and rehearses what she will say to Sam, the grocer. It's not clear what she's planning as she contemplates her possible jail time. However, when she gets to the grocer, we see the behavior of someone who has faced no trauma or tragedy. Mary acts as if nothing ever happened. She orders some food, chats with Sam, and takes the food back to the house.

As she's walking back, the narrator reinforces her new storyline, telling the reader, "she must cook it good, and make it as tasty as possible because the poor man was tired." She is creating a new narrative that completely contradicts the fact that she just killed her husband. Her husband is not tired; he's dead! She is rewriting herself out of the narrative.

When she returns to the house, she "discovers" her husband on the floor, presumably dead, and reacts as such. She calls the police and tells them her new story of grabbing some extra food for dinner and returning home to find him injured on the floor. Now, back to the grocery shop.

Mary's trip to the grocery shop is important for a few reasons. First, it gives her a possible alibi in that someone now knows where she was during the time of the murder. Sam can attest to the fact that Mary was her "normal" self and picked up some of her husband's favorite foods. The trip also helps her push the food onto her guests, which in turn destroys the murder weapon and any evidence of her guilt. She has this large meal prepared that is reminding her of her newly deceased husband, and she guilts the cops into eating all the evidence. The trip to the grocery shop, while it contradicts the reality of her behaviors, becomes the linchpin in her manipulative plan.

The most basic answer to this question is that she needed vegetables to go with the dinner she was cooking, a leg of lamb.  Because it was Thursday, Mr. and Mrs. Maloney usually went out to dinner, so she hadn't planned to cook.  But, as she tells the grocer, "'Patrick's decided he's tired and doesn't want to eat out tonight.'"  

The real reason Mary goes to the grocer is to establish an alibi for the death of her husband.  Before she left the house after hitting her husband over the head with the frozen leg of lamb, she put the leg of lamb in the oven to begin cooking.  Since she had not originally planned to eat at home, she needed to make it look like they had changed their plans, so she left the house to get the vegetables.  She even takes a moment before she leaves the house to practice what she will say to the grocer to make it seem more natural.  Thus, when she returns home and "discovers" her husband has been killed, her alibi is solid that she was out when he was killed.  And, since the murder weapon is being cooked, the detectives have no connection between her and the murder.  

After Mary spontaneously kills her husband by hitting him over the head with a frozen leg of lamb, her mind begins to work very rationally and methodically. It is paramount, to her thinking, to protect her unborn baby, and going to prison for murder will not achieve that goal.

She realizes that she will need an alibi to cover up the fact that she is the murderer. Therefore, she has to establish that she was out of the house at the time of the murder. She carefully dresses and heads out to buy some groceries for dinner so that people will see her and support her alibi that she was not home when her husband was killed by an "intruder."

She is able to be very calm and act normally while she is out shopping. When she comes home, she pretends she has stumbled on the murdered body of her husband and calls the police. When they arrive, she is able to deflect any shock she is feeling in the aftermath of killing her husband to the shock of finding him dead.

She also has the foresight to cook her murder weapon, a frozen leg of lamb, and feed it to the officers sent to investigate.

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.