The reason that Mrs. Mary Maloney kills her husband is that she is in a state of shock after his announcement that he is leaving her. If you had told Mary that she was going to kill her husband the day before the event took place, she would have told you that you were quite mad. At the start of Roald Dahl’s short story, you could not hope to meet a more dutiful wife than Mary.

When her husband arrives home, however, it quickly becomes apparent that it is not going to be a typical evening in the Maloney household. Having been awkward and stilted with his wife, Mr. Maloney finally admits that he has something to say. While Dahl does not spell out the contents of the conversation that Mr. Maloney has with his wife, it is apparent that he has told Mary, who is pregnant, that he is leaving her.

I would argue that Mary’s decision to hit her husband’s head with the frozen leg of lamb surprised her as much as anyone else. Her previous thought had been that “they would have lamb for supper.” This tells us that in the moments leading up to her using the leg of lamb to deliver a fatal blow to her husband’s head, she genuinely intended to cook it for their dinner. She had no premeditated plan to use it as a murder weapon. Therefore, Mary Maloney killed her husband because she was in a state of shock following the announcement of his betrayal.

In his play The Mourning Bride, William Congreve includes the following well-known lines:

Heav'n has no Rage, like Love to Hatred turn'd,

Nor Hell a Fury, like a Woman scorn'd.

The second line is often quoted as a proverb, but both lines are relevant to Mary Maloney's killing of her husband in "Lamb to the Slaughter." Her love turns suddenly to hatred, an alteration that happens so quickly that she herself seems to be unaware of it until she has actually killed him. Her rage is violent and automatic, as the frozen leg of lamb falls on Patrick's head like a thunderbolt from heaven.

This sudden violence from a woman who seemed unusually placid and serene until this point is caused by Mary's reaction to Patrick's infidelity and desertion. She is "a woman scorned" because Patrick has decided to abandon her, even while she is pregnant. There is something extreme about Mary's devotion to her husband even at the very beginning of the story:

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.

This seems more like the infatuation of a teenager than the stable love of a married woman. It is because her love for Patrick is so extreme as to be unbalanced that her reaction to his infidelity is equally extreme. She adored him with such intensity that she could not bear his abandonment.

Mary Maloney kills her husband because he has just told her he is leaving her. He explains the divorce as a done deal and says he will provide for her but offers her no negotiating room in terms of them staying together. He dictates to her what will be.

Mary is a heavily pregnant housewife who clearly devotes herself to her husband and her home. She did not see this new situation coming and therefore is in a state of shock. She is not in touch with her feelings at this moment, so she goes down to the basement to get a frozen leg of lamb, telling herself she will cook it for dinner. When she comes back up, her husband tells her not to cook for him, that he's "going out." Then, in what seems to be an almost out of body experience, in which she does not allow herself conscious awareness of what she is about to do, she whacks her husband on the back of the head with the leg of lamb. It is frozen so hard it is like a steel rod, and it kills him.

Mary seems quite detached from her murder and responds to it logically and dispassionately. We can imagine, however, that as a woman living in the 1950s and very invested in the role as homemaker and mother-to-be, the news of the divorce was too much for Mary to bear.

This question can be answered through a close reading of the text; Dahl doesn't directly state the reason, but it can be inferred from clues given.  If you look carefully at the story, it indicates that Mary was waiting contentedly for her husband to come home from work.  He does, and she waits  on him hand and foot, wanting to please him and make him happy.  However, he seems a bit distracted this evening, and drinks his entire cup of whiskey all at once, and is nervous.  Finally, he tells her that he is leaving her.  Roald Dahl writes,

"And he told her...he added... 'Of course I’ll give you money and see you’re looked after.  But there needn’t really be any fuss.  I hope not anyway.  It wouldn’t be very good for my job.'"

These lines hint at the fact that he has just relayed the horrific news that he didn't love her anymore and didn't want to be with her. Dahl doesn't relate the exact words that he says, but we can infer it from the fact that he says he will give her money, that he doesn't want any fuss, that it is bad news, etc.  And, Mary's shock at the news also implies that her husband, who she was very happy with, appears to be abandoning her and their unborn child.

This is reason enough for her to lose it.  In a daze, she goes down to get meat for dinner, and before she even realizes what she has done, she bashes her husband over the head with the frozen leg of lamb.  The reason is her shock and dismay at a man who she loved so dearly betraying her and leaving her and her baby to fend for themselves in the world.

I hope that those thoughts clear things up a bit; good luck!