Mary's behavior changes drastically throughout the story. Please explain how and why that happened.
Love can turn to hate, and it often does so in marriages. The surprising thing in "Lamb to the Slaughter" is that Mary's love for Patrick turns to hate so suddenly. Her action in killing her husband with the frozen leg of lamb is probably a surprise even to her. She seems to be acting on a blind impulse. If she hadn't been holding the leg of lamb, she probably wouldn't have committed the murder. Her impulse is probably caused by her sudden and complete disillusionment. She loved her husband. She was expecting a baby in a few months. She thought she had a happy home. She must have believed that Patrick loved her as much as she loved him. Then in just a few minutes he destroyed all her illusions, and at the same time he must have destroyed her love.
"This is going to be a big shock to you, I'm afraid," he said. "But I've thought about it a good deal and I've decided that the only thing to do is to tell you immediately." And he told her. It didn't take long, four or five minutes at most, and she sat still through it all, watching him with puzzled horror.
Characters don't really change. They evolve. Mary's violent reaction to Patrick's shocking betrayal must have been like uncorking a bottle and releasing a whole cloud of pent-up emotions and previously unsuspected character traits. Mary seems to become a different woman, but actually she is the same woman with multiple depths or dimensions exposed. We readers accept this new "liberated woman." Significantly, the author Roald Dahl doesn't try to explain her motivation. We don't question the possibility that she could have changed so radically. We find ourselves thinking along with her. How is she going to get out of this situation? What would become of her unborn baby?
It certainly shouldn't be surprising that someone kills someone else in a fit of rage. It must happen all the time. It has often been called "the urge to kill." And it shouldn't be surprising that the perpetrator doesn't want to get punished for it. Since we are the only witnesses to Mary's crime we are like accessories after the fact, so to speak. We don't like Patrick. We don't blame Mary for clobbering him. We want to see her get away with her crime. Maybe she shouldn't have done it, but what's done is done.