clear that she had not planned her crime, regardless of the fact that she was swift and clever in covering it up. Hence, let's analyze her state of mind prior to the crime, at the moment of the crime, and after the crime.

Prior to the crime, the surprise of her husband's news made her have a huge lapse of confusion.

When she walked across the room she couldn't feel her feet touching the floor. She couldn't feel anything at all- except a slight nausea and a desire to vomit. Everything was automatic now-down the steps to the cellar, the light switch, the deep freeze, the hand inside the cabinet taking hold of the first object it met.

This is clearly evidence of how this caused her a shock to the system. As she is pregnant, she is already in a delicate state. Imagine being told that your entire world will be destroyed right when you are about to become a mother, and right when your world seemed as if it was perfect. The state of daze that she felt could not have possibly rendered her able to make any plans at all and try an consequence. There is no sign of malice aforethought.

The second piece of evidence is the speed with which the act occurred during the crime. If it had been planned, Mary would have prepared the setting as well as the scene of the crime for it to be perfect. Although it was indeed perfect, it was all coincidence and situational irony. Regardless, Mary used "that one object" she grabbed during her daze and the energy that the whole situation caused in her, added to what could be argued as her raging hormones, made her snap.

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 might just as well have hit him with a steel club.

This could very well be another evidence of snapping, and not planning a murder. She is still in a daze. She is still in shock. Now she is shifting to survival mode, as she is clever enough to know that something terrible has just taken place and that she is guilty of it. Whether planned or by snapping, she has still killed a man. Notice that she takes no redeeming value out of the crime. She simply has no reaction.

The actions after the crime show survival, rather than malice. The calling the cops, the serving of the lamb, going to the grocers. This is not pre-meditation, but post-traumatic activity. This is why it is so odd that, at the end, Mary giggles (as if crazed) when she realizes what is happening. It is all happened in hours. It is clear that she may have lost her mind.

It's very much a crime of passion. Mary's whole world is crashing down about her ears. Her husband, to whom she's given such undivided loyalty and devotion for many years, has told Mary he's leaving her. Mary's whole life has been devoted to caring for her husband; she cannot conceive of any kind of meaningful life without him. So she lashes out, picking up the nearest blunt object to hand—a leg of lamb—and hitting her husband with it.

Once Mary has committed the murder, she shows herself remarkably adept at covering up her crime. But this isn't a sign that it was premeditated in any way. Just as the murder was improvised, so too is the ensuing cover-up. Mary's entered a different world now, one with completely different rules and standards of behavior. She didn't ask to be part of this world, but she's already shown herself more than capable of living and thriving in it.

Good questions!

I believe that the murder was a crime of passion. I do not believe that Mary premeditated Patrick's death. In fact, it wouldn't be called murder anymore with a crime of passion. manslaughter would be the more appropriate term. The text says that the noise of Patrick hitting the floor sort of snapped Mary awake, which is why I don't think that Mary pemeditated anything. 

The violence of the crash, the noise, the small table overturning, helped to bring her out of the shock. She came out slowly, feeling cold and surprised, and she stood for a few minutes, looking at the body, still holding the piece of meat tightly with both hands.

Is Mary cunning? Yes, I believe that she is quite cunning. She does a great job of establishing an alibi that a witness can corroborate.  Then she finds a way to get rid of the murder weapon forever by having the police eat it. That's cunning.