She cracked the back of his skull.

She made an alibi to cover it up

She had intentions and followed through with them

In the story, I Know Mrs. Maloney committed murder because she directly stated to herself that she committed murder and she knew that she had to hide the evidence and went out of her way to make a cover up for her actions

She did in fact kill him because there is no way a person can survive after getting their skull cracked open. And even if she didn't, why would she go as far as to make an alibi for herself if she didn't think he was dead and not just knocked out?

Mrs. Maloney murdered her husband with a leg of lamb after finding out that he wants a divorce. Mary was, understandably, distraught at the news, and may have gone into a kind of shock. The text suggests that she did love Patrick, and couldn't bear the thought of him leaving. So, she did the most logical thing (in her mind): killed him. The passage states, "...and when she saw him lying there on the floor, it really was a
shock. All the old love for him came back to her, and she ran over to him, knelt down beside him, and began to cry hard. It was
easy. No acting was necessary." The fact that Mrs. Maloney cooks the leg of lamb, which is eaten by the police officers (effectively destroying the murder weapon, and evidence of her involvement in the murder), makes this the perfect crime. However, it is worth noting that, scientifically, murdering someone is meant to trigger feelings of guilt and responsibility; even people who have carefully planned the murder will be sent into a kind of panicked shock once the act is done (assuming it was the first time they had killed). Knowing this, the reader may assume that Mrs. Maloney is in shock herself, that the depth of her actions have not truly registered yet. After all, when the story began, she was nothing but an ordinary housewife, but by its end, she was a murder. "And in the other room, Mrs. Maloney began to laugh." (Though she is depicted as almost psychotic in this scene, her reaction must be examined to the fullest extent, nothing about this story is black and white).

Instead of confessing, she carefully plans an alibi, going to the grocery store to create witnesses. When the police arrive, she deceives them by acting like a grieving wife and even destroys the evidence by cooking the murder weapon and serving it to the officers. Her calculated actions, from planning to cover-up, prove that she was fully aware of her crime and worked to avoid punishment.

To defend the claim that Mrs. Maloney commits murder in the story, consider the following evidence. Mrs. Maloney strikes her husband with a leg of lamb, delivering the fatal blow. Her emotional state and motive, triggered by her husband's announcement that he's leaving her, suggest premeditation and intent. Furthermore, there's no indication of self-defense or justification for her actions. Instead, Mrs. Maloney conceals evidence by cooking the leg of lamb and serving it to the police, demonstrating deliberate deception. Her calm and calculated behavior after the murder also implies a level of control, supporting the claim that Mrs. Maloney commits murder in the story.