Mary Maloney seems like the kind of woman who is often described as a "doormat." She is totally dependent on her husband. She is extremely domestic and content to stay at home and wait for him to return from work. The fact that she is six-months pregnant only makes her seem more domesticated, complacent, and dependent. A woman like this could easily become boring to a husband who had an interesting job in the outside world, such as that of her husband Patrick who is a policeman. She doesn't realize that her love, dependency, submissiveness, attentiveness, and all such other aspects of her domestic character can become intolerable to her husband.
Mary Maloney's character brings to mind the short story "The Chaser" by John Collier. In that story the old shopkeeper tells his young customer that he will eventually get so tired of being the husband of an adoring, possessive wife that he will want to get rid of her.
"She will care intensely. You will be her sole interest in life. . . . She will want to know all you do. . . . 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 terrrifed. She will think you are killed, or that some siren has caught you."
(John Collier's story is thoroughly discussed in eNotes. See the reference link below.)
Mary Maloney in "Lamb to the Slaughter" has the wrong idea about marriage. She believes that she can retain her husband's love by being his devoted slave. Patrick Maloney seems to be the sort of man who is not in touch with his feelings, but who can come to devastating conclusions after a long period of silent endurance. Instead of feeling more closely bonded to his wife by the baby she is expecting, he feels all the more suffocated by the addition of another emotional burden, another link to his chain.
Mary, too, has not been in touch with her true feelinigs. She is the type of person who represses angry feelings and lets them build up until there is danger of an explosion. She becomes more her genuine self when she realizes that all her love and care have had exactly the opposite effect to what she expected.
"For God's sake," he said, hearing her, but not turning round; "Don't make supper for me. I'm going out." 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 becomes a much more interesting character after that reaction. She loses her dependency and starts thinking for herself. We may not approve of wives killing their husbands, but we can still admire her self-sufficiency, which was present all along without her being aware of it. She stalls about calling the police in order to establish an alibi. She puts the frozen leg of lamb into the oven and turns it up to high, then goes to Sam the grocer to establish that she was absent from home when her poor husband was murdered by some intruder. Finally she serves the cooked leg of lamb to the policemen who are searching all over the premises for the murder weapon.
Why do we want to see Mary Maloney get away with murder? Her response to her husband's rejection seems justified, and we enjoy stories in which "the worm turns."