We can describe Mary's decision to kill her husband in the following way: She was fueled by hurt that he was leaving her.
How to describe the decision
In the story, Lamb to the Slaughter, Mary was caught in a stream of grief, betrayal, and desperation, and she grappled with a future she could not imagine without her husband. The weight of his absence pressed down on Mary, suffocating her with a sense of isolation and injustice.
So we can conclude that Mary's decision to kill her husband was fueled by hurt that he was leaving her.