Can anyone think of a continuation of the story "Lamb to the Slaughter?" Or a "prequel?"
Sure. I love that story. Continuing it is a bit of a challenge, though, because it seems to be so well self-contained. I think I have an idea, though.
In the story, the reader is never told why Patrick wants to leave his wife. We miss that conversation. The most reasonable idea, though, is that he has taken another woman on the side. This seems to fit well with the "silent rage" the woman felt when when killed her husband...only a cheating loved one can raise that level of blind hate.
With this in mind, think about the ending of the story. She seemingly gets away with it. But what about this other woman? What happens when she comes forward to the police and says "Patrick was going home to tell his wife about our affair, and now he has turned up dead!" The police, their suspicion aroused, reinvestigate the wife but are unable to pin the crime on her. As a result, the mistress decides to take a bit of revenge. She creates the "perfect murder plot" herself and exacts it on the "grieving widow" (after the baby is born, of course.) Perhaps she becomes the widow's nanny (paid for with the life insurance money the widow was paid.) In the end, she even "adopts" Patrick's child.
Anyhow, it's an idea. It would be up to you to find a good "perfect crime" worthy of "Lamb to Slaughter." Good luck!