In any case, the fact that she “simply walked up behind him” and unhesitatingly winds up the lamb leg before bludgeoning him “as hard as she could” demonstrates volition, determination, and silent rage—traits she does not display as the traditional housewife earlier in the story.

After hitting her husband, Mary just steps back silently and watches him fall to the floor. Only when his crashing fall overturns a table does she come “out of the shock…. feeling cold and surprised” as if she was in a trance. The fact that she is “surprised” suggests that her actions are not premeditated. The wording indicates spontaneous and temporary insanity.

She remains eerily calm while standing over his body; instead of screaming or panicking, she tells herself, “All right… So I've killed him.” With this matter-of-fact statement, Mary may be revealing her belief that he got what he deserved. After all, his destruction of their seemingly perfect marriage—at least to outsiders—and desertion of her would be reprehensible in 1950s American society.

The words that you could use to describe Mary Maloney would likely look quite different as you follow her character transformation.

At first, she is devoted. She sits and waits for her husband's return, prepared to spend a wonderful evening with him. When he arrives, she is quick to hang up his coat for him and makes his favorite drink. She allows him space to unwind without talking because she knows this is important to him after work. She admires the shape of his mouth as she quietly waits for him to engage with her.

Not much later, she is confused. The man to whom she devotes herself delivers news that is shocking. Though the details are omitted, it is assumed that Patrick Maloney has been having an affair and is leaving her. Mary is so confused that she simply tries to pretend she's imagining the entire conversation, and she robotically begins preparing their dinner in a desperate state of confusion.

When Patrick insults her once more, telling her that he's already told her not to cook for him because he is "going out," presumably to the other woman, Mary is unflinching. Holding a frozen leg of lamb, she doesn't hesitate as she swings it high and crashes it down into her husband's skull.

As she surveys the scene that follows, Mary is methodical. She immediately begins generating an alibi, calmly washing her hands and thinking quickly about how to remove herself from the scene and simultaneously place herself somewhere with witnesses. Thus, she heads to the neighborhood grocery and strikes up a conversation with the grocer.

In the end, Mary is relieved to the point of giddiness. She's gotten away with murder by feeding the murder weapon to the detectives themselves. As they desperately search for clues, Mary shows no remorse toward her actions. She has saved herself and her unborn child with her own quick thinking and with her careful attention in covering the crime.

Through the course of the story, the reader gets the impression that they are witnessing a transformation in Mary, from docile wife to murderer. However, we are given very little information about what the Maloneys married life was really like before that fateful day. Because Patrick has been having an affair and had successfully hidden this fact from his wife, she does not seem especially perceptive. The author’s early descriptions of her character are so different from her subsequent behavior, however, that the reader wonders if she had been repressing her knowledge and was hiding a simmering rage. If that were the case, we might go back and re-evaluate Dahl’s first glimpses of the wife.

Mary is apparently “trusting” in that she never suspected Patrick’s affair. She is definitely “pregnant,” and her maternal feelings motivate her cover-up of her attack. Mary comes across as “passive” in her relationship to her husband, but she shows that she is “calculating” when she goes to the grocery store to concoct an alibi. She also proves that she is “devious” as she misleads the detectives both about the crime itself and by destroying the evidence.

Mary Maloney is a doting wife. She does not work and waits at home for her husband ensuring the house is clean and dinner will be served on his timeline.

In the same vein, Mary could be described as selfless. Not once does the reader see evidence of her own needs. Pregnant Mary focuses solely on her husband, counting down the hours and minutes for him to come home.

Mary is a faithful wife. She is deeply devoted to her husband and his needs. After she learns the terrible news, she continues to push forward in hopes they can move past this. She cannot bear to think about losing her one true love.

However, Mary ends up snapping and killing her husband in a clever way. Here, she could be described as cunning. She not only gets revenge on her husband, but she tricks the cops into eating the murder weapon.

This reaction to her husband's decision to leave her reveals her manipulative nature. Yes, she is a devoted wife, but Mary quickly shifts her personality from a murderous and vengeful woman to a poor, innocent victim.

I would say that the majority of your adjectives would need to come from the beginning of the story, because Dahl is very careful to build up a precise nature of Mary's character to prepare us for the massive situational irony that occurs in the middle of the story. The adjectives I would use to describe her would therefore be loving, devoted, subordinate, serving and obsessed. This would be from the way in which she greets her husband and the high level of love that she has for him. Consider the following quote:

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.

Note the way in which the centre of Mary's existence is said to be her husband, and the way that she waits on him and does what he wants without even thinking of her own needs and wants is rather disturbing in its slavish devotion. Of course, all of these qualities makes her shift into a calculated master criminal all the more surprising.

The personality traits of Mary Maloney in "Lamb to the Slaughter" are meekness, passivity, self-effacement, extreme deviousness, and calculation.

On the face of it, it might seem that these characteristics don't sit well together. But as the story progresses, we find that all of them are indeed combined in one person.

When we first meet Mary, we see a mousy, timid housewife, a woman who wouldn't say boo to a goose. Subscribing as she does to traditional gender roles, Mary accepts being subordinate to her husband, to whom she is utterly devoted.

And this is how she would almost certainly have remained had her husband not told Mary that he was leaving her for (what we assume to be) another woman. But as it is, another set of character traits comes to the surface after Patrick drops the fatal bombshell. For once Mary has murdered Patrick by hitting him over the head with a frozen leg of lamb, we see a whole different side to her.

Now we see a devious woman able to cover her tracks and present herself to the investigating police officers as a grieving widow rather than the murderer that she is. In covering her tracks, Mary also shows herself to be cold and calculating, able to come up with a convincing alibi that will keep her out of the frame for the murder that she's just committed.

Mary is a completely devoted wife. She adores Patrick and waits on him like a maid. She is doting, attentive, and effusive. This may be why Patrick decides to leave. We are never given a reason. He may have been having an affair as well. The author never makes this clear. As for Mary, she feels completely comfortable and satisfied with her life with Patrick. She is six months pregnant and it would appear that she has complete faith that their future family will be a happy one as well. She is so happy, dependent upon, and we might even say addicted to Patrick that when he informs her he is leaving, she cannot handle it. 

The author doesn't give a word-for-word dialogue of how Patrick tells Mary he is leaving her. But by the end of it, she knows the marriage is over and there is no hope of reconciliation. She is stunned and then reacts quickly. In her shock (or conscious reaction), she kills him. Immediately after this, she becomes a clear-thinking detective, determined not to be caught. She carries out her plan to perfection. When the detectives eat the evidence, she laughs in the other room. The seemingly perfect, devoted wife reveals a sinister side in this last line. She is a much more complicated and mysterious character than the opening paragraphs would suggest.

In the beginning of Roald Dahl's "Lamb to the Slaughter," Mary Maloney is described as a stereotypical housewife. She is happily pregnant and is happy when it is approaching time for her husband to come home. All of her actions are that of a woman who is blissfully content with her station in life as a dutiful and loving wife. 

There are, however, a few descriptors that serve as forewarning that Mary may not be as stable, calm and loving as she seems. Roald Dahl uses expressions like "curiously tranquil" and noted that her eyes seem "larger, darker than before." It almost suggests that she was in a dreamlike state in the beginning of the story. Whether it was due to the pregnancy or not is impossible to say. If she was in a dreamlike state instead of actually tranquil and steady, it would explain why the shock of her husband's intent to divorce her caused her to act the way she did. It would also explain why now, suddenly awake, she is able to transform from a loving wife into a woman cleverly covering her tracks when the murder itself seemed sudden and accidental. 

Mary is described over and over again as peaceful.  Although Dahl will suggest in some parts that she is happy, and in others that she is shocked or sorrowful, overall Mary is peaceful woman, hard to ruffle.  She is calm when she kills her husband, calm when ordering groceries, calm when the police arrive.  She is not overally affected by the world around her, which makes us wonder is she exceptionally well-adjusted or just exceptionally disturbed.  Here are Dahl's own words:

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.

And then, after she kills him, she barely reacts except to plan ahead:

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.All right, she told herself.  So I’ve killed him.It was extraordinary, now, how clear her mind became all of a sudden. 

The reader learns several things about Mary Maloney in the early parts of the story.  If you are referring to something very concrete about her that the reader learns, then I believe the most important detail is the fact that Mary is six months pregnant.  It's a key detail to the story, because after she kills Patrick, Mary is perfectly willing to take the punishment.  But she doesn't know what will happen to the baby, so she decides to cover up the killing for the sake of her unborn child. 

As the wife of a detective, she knew what the punishment would be. It made no difference to her. In fact, it would be a relief. 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.

The reader also learns that she is desperately in love with Patrick.  She's practically a "Stepford Wife."  Mary is the quintessential doting wife.  She is sitting in the front room eagerly awaiting the arrival of her husband.  He is the reason for her existence.  

The room was warm, the curtains were closed, the two table lamps were lit. On the cupboard behind her there were two glasses and some drinks. Mary Maloney was waiting for her husband to come home from work. 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.

Mary Maloney is a happy, contented woman who feels secure and confident in her life.  Roald Dahl describes,

"There was a slow smiling air about her, and about everything she did."

She has a husband that she loves, a routine that she can trust and count on, and joy in the coming baby.  She feels this way because as long as she has been married, the routine has been the same, and she takes comfort in it.  She figures that if that routine exists, all must be well.  She takes the routine as evidence of her happy life and marriage.  So, imagine that you were perfectly content, and confident in the happy life that you lead, and the person you love and worship most in the world, drops a bomb on you by saying they don't love you anymore and are leaving you.  Leaving you, and the baby, to be abandoned and dejected.  This would be totally unexpected, and a complete shock.  So shocking in fact, that it might jar you right out of your normal personality, and bring forth your survival instincts.  Mary reacts without thinking, from the shock, and ends up with a dead husband on her hands.  She realizes what she has done, and is willing to accept the consequences, but, she worries about the child:

"As the wife of a detective, she knew quite well what the penalty would be.  That was fine.  It made no difference to her.  In fact, it would be a relief.  On the other hand, what about the child? "

So, she uses what she has learned from being a detective's wife, and sets up the perfect escape from the crime.  She, in defense of her child, does what she has to do.  So, she changes from a rather passive, contented woman to an active woman who takes charge to protect her own, and the circumstances are what prompted that change.  I hope those thoughts help; good luck!