Why does Mary Maloney insist the police eat the leg of lamb in "Lamb to the Slaughter"?

Mary's husband, Patrick, announced that he was leaving her (or a divorce).  For a few minutes after that, Mary is operating on autopilot.  She moves about the house in a daze, and she decides that she is still going to cook dinner.  Mary gets the lamb, and she announces her intentions to Patrick.  He, very callously, announces that he doesn't want any dinner, because he is going out. 

"I've already told you," he said. "Don't...

Mary's husband, Patrick, announced that he was leaving her (or a divorce).  For a few minutes after that, Mary is operating on autopilot.  She moves about the house in a daze, and she decides that she is still going to cook dinner.  Mary gets the lamb, and she announces her intentions to Patrick.  He, very callously, announces that he doesn't want any dinner, because he is going out. 

"I've already told you," he said. "Don't make supper for me. I'm going out."

Mary snaps.  She hits Patrick over the head with the leg of lamb.  The blow kills him.  

Mary admits that she knows what the punishment will likely be for her, but she doesn't know what will happen to her unborn child.  She isn't willing to risk finding out, so she makes a plan to get away with the murder.  

It was extraordinary, now, how clear her mind became all of a sudden. She began thinking very fast. As the wife of a detective, she knew what the punishment would be. It made no difference to her. In fact, it would be a relief. On the other hand, what about the baby? What were the laws about murderers with unborn children? Did they kill them both -- mother and child? Did they wait until the baby was born? What did they do? Mary Maloney didn't know and she wasn't prepared to take a chance.

In order to do that, Mary needs two things.  First, she needs an alibi.  She does that by going to the grocery store and speaking to the clerk.  Second, she needs to get rid of the murder weapon.  Mary does this by cooking it and making sure the police eat all of it.  Without a murder weapon, the police have no physical evidence to tie Mary to the murder.