Roald Dahl makes this ordinary woman sympathetic by telling the whole story from her point of view and by giving her a problem with which the reader can relate. These are standard techniques for getting the reader involved in the story by getting the reader involved with a character. The second paragraph can serve as a good example of how Dahl narrates the story through Mary Maloney's point of view.

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. As she bent over her sewing, she was curiously peaceful. This was her sixth month expecting a child. Her mouth and her eyes, with their new calm look, seemed larger and darker than before.

The anonymous narrator can tell what Mary is doing and can also go into her mind and tell what she is thinking and how she is feeling. It is noteworthy that when her husband Patrick comes home, the narrator does not switch to his point of view but represents his thoughts and feelings through Mary's perceptions of what he says and does.

"Tired, darling?"

"Yes," he sighed. "I'm thoroughly exhausted. And as he spoke, he did an unusual thing. He lifted his glass and drank it down in one swallow although there was still half of it left. He got up and went slowly to get himself another drink.

After she murders her husband with the frozen leg of lamb, the reader identifies with her because of her problem. She doesn't want to go to prison or to be executed. Maybe she deserves punishment, maybe not. But in any case, the reader is kept in her point of view and has to identify with her as she plans her alibi and puts on a big act of grief, shock, and innocence for all the police when they arrive.

We will identify with almost any kind of character if we are confined to that character's point of view. For instance, we identify with Fyodor Dostoevsky's Rodion Raskolnikov in Crime and Punishment after he has murdered two women with an axe. The unnamed protagonist in Jack London's story "To Build a Fire" is a brutal, repulsive man but we identify with him because we are confined almost entirely to his point of view, except for a few moments in which we see him from the point of view of his dog. Both these men have serious problems. Raskolnikov doesn't want to get caught and sent to Siberia. The Chechaquo in "To Build a Fire" doesn't want to freeze to death in the snow. We sympathize and identify with them because we are in their points of view and because we can relate to their problems.

Even at the end of "Lamb to the Slaughter," when all the policemen are devouring the leg of lamb and talking about the case, we remain in Mary's point of view and are both amused and relieved as her problem is solved. The story ends with a line which shows that all the policemen's dialogue has been overheard by Mary and therefore that we have not left her point of view.

"Have some more, Charlie."

"No, we'd better not finish it."

"She wants us to finish it. She said we ought to eat it up." 

"That's a big bar the murderer must have used to hit poor Patrick. The doctor says the back of his head was broken to pieces.

"That's why the weapon should be easy to find."

"Exactly what I say."

"Whoever did it, he can't carry a weapon that big around with him."

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

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

And in the other room, Mary Maloney began to laugh.

How does Roald Dahl evoke sympathy for Mary Maloney?

When the reader first meets Mary Maloney, she seems like a bland, insipid person. Her concerns are humdrum and routine: she is a housewife making things nice for her husband’s arrival home from work. Roald Dahl engages the reader’s sympathy both by making her husband such an unsympathetic character and, ironically, by showing Mary’s cleverness and determination as a killer who commits the perfect crime. Mary, we learn, is pregnant, and her monstrous husband is walking out on her. His cavalier attitude toward parenthood, combined with the fact he has a mistress, renders him intolerable. Just as the reader wonders what will become of Mary, her true character pops out. We see that she is a decisive person as she brains him with the frozen meat. Despite being horrified, the reader now starts to root for Mary to get away with it. Thanks to her cleverness and apparent generosity to the policemen, she does.

The opening three sentences of the story establish the goodness and dutifulness of Mary Maloney.

The room was warm and clean, the curtains drawn, the two table lamps alight—hers and the one by the empty chair opposite. On the sideboard behind her, two tall glasses, soda water, whiskey. Fresh ice cubes in the Thermos bucket.

She awaits her husband's return from work with thoughtful preparations, and when he does arrive, she allows him to unwind at his own pace. Mary "loved to luxuriate in the presence of" her husband, and readers may admire her love and devotion to him and the way she is attuned to his needs.

Once Patrick has told his six-months-pregnant wife that he is leaving her, readers may feel sympathetic toward Mary's almost robotic reaction. Patrick is an unsympathetic character, and his rejection of Mary and their unborn child makes him seem monstrous. Since the story is fictional, readers may allow themselves to feel that Mary's retaliation and extreme cleverness in covering up her crime are acceptable and ironically amusing. Mary Maloney is more than a simple, demure wife; she has the temerity to meet Patrick's cruelty with a game-ender.

How does Roald Dahl make us sympathize with Mary Maloney in "Lamb to the Slaughter"?

There is no question that the character of Mary Maloney in Roald Dahl's short story "Lamb to the Slaughter" does make us feel sympathy for Mary, for a number of reasons.

First, it is easy to recognize that Mary has reached a very desirable comfort zone. She evidently loves her current status as a married woman, as a future mother, and as a housewife. Her home evokes a feeling of warmth and peace when we first read the story. We know that Mary is a happy woman who takes good care of her home. Now, she is devotedly waiting for her husband, a police officer, to come back from work so that she can make him dinner.

Right there in that first description we immediately can connect with Mary. How many women would not dream of an idyllic marriage like that? Even if we cannot connect with Mary as a wife, we still sympathize with the fact that she is an expecting mother. We all wish to see our own mothers in a happy state of affairs like that. We all would want the same peace and tranquility, not to mention the comfort and security, that Mary felt in her own home.

Roald Dahl switches that mood quickly, and we all witness how Mary's husband returns home from work, drinks more than usual, refuses his dinner, and then basically tells her that he is leaving. All that shatters the immediate bond the reader forms with Mary, and perhaps even makes the reader feel as sad as a "real-life Mary" would have felt. Roald Dahl treats the theme of domestic neglect using the emotional triggers that create a sense of happiness to sadness, from joy to pain, and from hope to hopelessness.

It is that hopelessness that Roald Dahl instills in his writing what makes the reader understand 100% how Mary's shock led her to snap and kill her husband. To have such a perfect world come down for no fault of her own seems a huge deal for someone who is expecting a child, and someone who would not want a stigma of divorce hanging over her. Mary does what any other woman in a deep state of shock would have done: To snap. It is hard not to sympathize with her....and even to excuse her behavior, even if it means that her husband is dead.