"Lamb to the Slaughter" could be read as a story of revenge because the reason behind the killing of Patrick Maloney was that his wife, Mary, reacted violently to the news that her husband was leaving her.

Keep in mind that Mary's reaction is also a result of shock. She pushed down her emotions and carried on as usual.

Her first instinct was not to believe any of it, to reject it all. [...] Maybe, if she went about her business and acted as though she hadn't been listening, then later, when she sort of woke up again, she might find none of it had ever happened.

Then, out of nowhere, she smacked her husband over the head with a frozen leg of lamb she had grabbed from the freezer in what could be classified as heat of passion. All happened quite quickly, and shortly after he delivered the news. Regardless, the act of violence was caused by the reaction to those news. 

The rest of the story, after the killing, actually reads more like a tale of  revenge. At this point, Mary is no longer reacting in the heat of passion; she is actively concealing her crime, and trying to get away with it. She is now acting for herself, and it is clear to her now that this will be a struggle for survival for her and her child; the same two individuals that Patrick so callously planned to abandon.

The way Mary acts suggests more that she thinks of Patrick's death, not as a crime committed by her, but as an obstacle to her freedom. As such, she will ensure that he will not get in her way. Now she is actively getting rid of Patrick for a second time.

Mary creates and supports an alibi by going to the market, speaking to Sam there, and making sure that he learns that Patrick is at her house, waiting for his dinner. 

The man turned and reached up behind him on the shelf for the peas. "Patrick's decided he's tired and doesn't want to eat out tonight," she told him. [...] "Then how about meat, Mrs. Maloney?" "No, I've got meat, thanks. I got a nice leg of lamb from the freezer."

Then, she plans to get rid of the murder weapon: by feeding it to those who will come and investigate the crime. Third, she will ensure that the whole thing is gone, and so is any suspicion about her. 

The end shows that, to Mary, this has moved from being a sad moment to an interesting one. She giggles as they eat the lamb! She has clearly gotten away with everything, and she is quite pleased with her results.