What is a symbol in A Good Man is Hard to Find? How does it advance or reinforce the plot?

Flannery O'Connor's short stories are teeming with symbolism, and A Good Man is Hard to Find is no exception. There is an overall theme of good versus evil in the story, and symbols push this theme forward. 
After the family has had their car accident, the sky is described as being both sunless and cloudless—clear and empty. It is almost as if the family is caught between night and day, which could translate to being caught between life and death (as they are about to be killed), or perhaps even good and evil. These people, the grandmother especially, have been put in a sort of neutral zone, where their goodness or innocence (whether real or a facade) cannot save them. 
The empty sky also connects to The Misfit. We learn throughout the course of the story that this man, like the cloudless sky, is empty and ambiguous, possessing no regard for what is good or evil. He is in control, just as the sky dominates the setting.

One salient symbol in "A Good Man is Hard to Find" is the shape of a cross into which the grandmother's legs contort after the Misfit shoots her.
This cross made by the grandmother symbolizes her redemption and moment of grace as she has looked at the Misfit and recognized him as one of her children. That is, she has come to know herself as a sinner through what is termed a "redemptive catastrophe." She dies for her sins just as Christ died for the sins of man, and she is redeemed. O'Connor's approach here recalls the idea of the philosopher Martin Heidegger, who was a strong influence upon her. Heidegger, who held a concept called Dasein, or "being-there," felt that death represents the moment when a man's existence becomes complete.
Certainly, for the grandmother grace comes to her in her epiphany at the moment of her brutal death. This is why the Misfit says, "She would have been a good woman if it had been somebody there to shoot her every minute of her life." At the moment of her death, she is redeemed because she finally has enough humility to recognize that she, too, is a sinner--she is "there" with the Misfit--and she abandons her self-righteousness displayed in the car and at Red Sammy's, and thereby receives grace.
This redemptive death acts as the denouement to a plot in which her hypocrisy has been exemplified in her criticisms of others (e.g. her racial remarks when she has set herself up as honest and righteous woman). At the end, she finally recognizes that it is she who has been the greatest sinner and hypocrite.