Based on her name, Mary, it makes the most sense that the author might be referencing Jesus's mother, Mary. The other possibility is Mary Magdalene. Both Biblical women had interactions with Jesus. Obviously Jesus's mother had interactions with Jesus. Mary Magdalene reoccurs throughout the four gospels as a follower of Christ. She is also the first person that Christ appeared to after his resurrection. The title of the story, "Lamb to the Slaughter," is referencing Christ Jesus. He is said to be the Lamb of God, was led to the crucifixion like a lamb to the slaughter. That's why it makes the most sense that the Mary from the story is referencing one of the two Marys that were close to Christ. Mary Magdalene saw Christ crucified like a slaughtered lamb. Mary killed her husband with a slaughtered lamb.