In an interview sometime in the 1950s, O'Connor described her writing very clearly,
'I have found . . . from reading my own writing, that my subject in fiction is the action of grace in territory largely held by the devil.'
Many literary critics argue that pursuing a story's meaning by trying to understand an author's intention is a waste of time. Where we have, however, O'Connor's very explicit statement about a theme she recognizes throughout her writing, we should try to ascertain whether that theme exists in O'Connor's stories. In this case, we need to ask ourselves if "A Good Man is Hard to Find" has, as a main theme, "the action of grace in territory largely held by the devil."
The "action of grace," I would argue, occurs at the end of the story when the Grandmother is trying to save her life as she pleads with The Misfit, and she initially clings to her vague arguments about what a good man is,
'I know you wouldn't shoot a lady! I know you come from nice people! Pray! Jesus, you ought not to shoot a lady.'
The Grandmother, with her shallow Christian virtues, tries to appeal to those virtues in The Misfit, not recognizing that The Misfit is actually in the midst of an existential crisis brought on by his lack of faith in Jesus--The Misfit is, in real terms, in the territory of the devil because he has no faith.
When The Misfit finally says that "if I had been there I would have known," he is expressing his despair at not knowing whether Jesus is worth believing in or not and, more important, his recognition that if he had faith, "I wouln't be like I am now." As he comes to this realization, "his voice seemed about to crack and the grandmother's head cleared for an instant." The Misfit's ultimate despair at having no faith opens the way for The Grandmother's "action of grace."
This superficial lady reaches out and touches The Misfit's shoulder and says
'Why you're one of my own babies. You're one of my own children!'
At this point, The Grandmother steps outside of her superficial self and feels true Christian compassion for The Misfit, and tries to comfort him as if he were a lost child rather than a merciless killer. The fact that The Misfit immediately kills the Grandmother is tangential to O'Connor's theme here. The action of grace occurs in spite of the fact that the devil still holds this territory.