Whichever the laws of her particular state, Mary Maloney would still have had a really bleak future had she been found guilty of murdering her husband in Lamb to the Slaughter.
First, she not only killed her husband, but her husband happened to be a police officer. That alone is a huge deal because she would be categorized immediately as a "cop killer" and the punishment that is applied for attacking (let alone KILLING) an officer of the law is twice as harsh.
Second, she concealed evidence. If any of the police officers that were eating the lamb that night had even suspected that the lamb was the murder weapon- and came forward with proof- Mary would have to also face charges for that as well.
Mary would have to go through the horrors of delivering her long-awaited baby in prison, perhaps never get out of prison, and she would have to find a trusted source to give her child under custody.
She would probably never see her child grow up, and her pretty and comfortable home would now be a cold prison cell surrounded by dangerous females that could be cold-hearted and mean to her.
Even worse, Mary could have ended up in death row and really NEVER even have the privilege of any visits from her child. Her family will probably shun her and she would have been all alone and eventually die unhappily.
When you think about it, you almost can understand why she had to come up with a plan.