What are the key themes that Flannery O'Connor explores in "A Good Man is Hard to Find"?

Key themes O'Connor explores in this story are grace and redemption. Despite a comic opening as an all-American 1950s family vacation saga, events in this tale soon turn grim. When the family car lands in a ditch on a deserted road, The Misfit, a killer on the loose, emerges with his gang. They begin killing the family members, finally leaving only the Grandmother. The Grandmother has been a flawed, annoying character since the start of the story, manipulative, racist, difficult to get along with, and more than anyone else, responsible for the family's dire predicament. Yet even this flawed woman is open to God's grace. She receives it at the point of death, when the Misfit comes out of the woods, wearing her son's shirt. Probably because he is wearing the shirt, she is able to see him as a human being, just like her son, and for a moment they connect:

His voice seemed about to crack and the grandmother's head cleared for an instant. She saw the man's face twisted close to her own as if he were going to cry and she murmured, "Why you're one of my babies. You're one of my own children!" She reached out and touched him on the shoulder.

The Misfit shoots her, but at the point of death, the Grandmother experiences a moment of grace and redemption. By seeing the killer as her child, she has offered him love, a completely unconditional love that transcends what he deserves. In Christian terms, this ability to feel love for a person you should hate, even if only for a instant, is called grace, something understood to be from God. It redeems us in Christain theology, turning us from sinners to people of God. 
Does this moment touch the Misfit? He recoils from it as if it affected him, as if bitten by snake. But we are left uncertain about him.
O'Connor, a Catholic writer and religious person, communicates through this story that God's redeeming grace is available to anyone, even in the worst of circumstances.