It all seems to depend on what you mean by "good." Mary was obviously a devoted wife and completely good in the sense of being faithful to her husband. But there was something about her that made him want to get out of the marriage, even though he was well aware he was doing a cruel and immoral thing. Dahl illustrates clearly what that "something" was.
"I'll get it!" she cried, jumping up.
"Darling, shall I get your slippers?"
"I think it's a shame," she said, "that when a policeman gets to be as senior as you, they keep him walking about on his feet all day long."
"Darling," she said. "Would you like me to get you some cheese? I haven't made any supper because it's Thursday."
"If you're too tired to eat out," she went on, "it's still not too late. There's plenty of meat and stuff in the freezer, and you can have it right here and not even move out of the chair."
"Anyway," she went on, "I'll get you some cheese and crackers first."
Mary is too attentive, too dependent, too needy, too loving, too devoted, too clinging, too anxious to please. Her husband doesn't like all this slavish devotion. He is obviously a different type of person. He is not used to sharing his feelings with other people, even including his wife. He works hard all day and has to deal with dozens and dozens of people of all types. When he gets home at night he would like to have a couple of drinks and just unwind. But Mary has been alone all day, thinking about him, and just waiting for him to return. Patrick is not having an affair. He just wants more freedom and more privacy.
"Lamb to the Slaughter" is reminiscent of John Collier's story about marriage, "The Chaser." In Collier's story the old man who sells love potions warns his customer that he will get sick of marriage in time and may want a "chaser" that will poison the woman he is now so anxious to marry.
"She will want to know all you do," said the old man. "All that has happened to you during the day. Every word of it. She will want to know what you are thinking about, why you smile suddenly, why you are looking sad....How carefully she will look after you! She will never allow you to be tired, to sit in a draught, to neglect your food. If you are an hour late, she will be terrified. She will think you are killed, or that some siren has caught you."
(By "siren" the old man means another woman.)
Unfortunately, what is true of the marriage between Mary and Patrick Maloney and true in what the old potion seller is foretelling is that marriages do tend to get stale. The honeymoon period does not last, in spite of the fact that young newlyweds are sure it will last forever. Having children changes things too. The fact that Mary is expecting to have a baby in about three more months is not a source of delight to her husband. It only means that he will be tied closer to this claustrophobic domesticity. Perhaps it is the prospect of having a crying baby in the house that drives Patrick to make the drastic decision to leave his wife. If he is going to get a divorce, it would probably be better to do so before the baby is born rather than afterward. But for him to divorce her at such a time seems so outrageous to Mary that she obeys a sudden impulse to kill him. The frozen leg of lamb is a convenient weapon; it might also be considered a symbol of the cozy domesticity that is driving Patrick crazy. We see that he drinks more than usual on that particular evening, working up the courage to tell his wife what he has to tell her. However, we can well imagine that if he did not ask for a divorce, and if he had continued to stay at home and adhere to the same routine for the rest of his life, he would start drinking more and more heavily when he came home, and in time he would have a real drinking problem. The marriage could only get worse.
Mary is a "good" wife, but she has not been a smart wife. She ought to have realized that there needs to be space in marriage. In his well-known book The Prophet, Khalil Gibran writes:
Let there be spaces in your togetherness, And let the winds of the heavens dance between you. Love one another but make not a bond of love: Let it rather be a moving sea between the shores of your souls. Fill each other's cup but drink not from one cup. Give one another of your bread but eat not from the same loaf. Sing and dance together and be joyous, but let each one of you be alone.