"A Good Man is Hard to Find" is a confrontation of between a grandmother with a rather superficial sense of goodness, and a criminal who embodies real evil. The grandmother seems to treat goodness mostly as a function of being decent, having good manners, and coming from a family of "the right people." What a contrast, when the grandmother encounters The Misfit, who seems straightforwardly evil, with little to no sense of guilt, and a genuine desire to do cruel or destructive things for their own sake. Understanding the motivations of The Misfit, and what "goodness" means by contrast, is one of the central puzzles of the story.
The Misfit has no sense of right and wrong, and for this reason doesn't feel any punishment can ever "fit" the crime.
The Misfit recognizes the grandmother's final gesture as good, and understands "goodness" to be the unconditional love given by divine grace.