One of the key themes of this story is the way in which it presents us with two characters who are defined by their many faults and sins. Both the grandmother and the Misfit, to varying degrees, represent humanity in all of its sinfulness. However, if we analyse the conversation that the grandmother has with the Misfit towards the end of this story, we see that grace, an incredibly important concept for Flannery O'Connor, is shown to operate in both of these characters, presenting them with a possibility of change.
Note how the grandmother reponds to the Misfit's desire to ascertain what Jesus did and didn't do. She experiences a moment of grace in an epiphany when she acknowledges a shared common humanity:
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.
Of course, the Misfit isn't her literal child, but this recognition of a shared humanity actually represents the grandmother's sanest moment in the entrie short story. She has been granted clarity and compassion before she dies. Note, too, how the Misfit has been changed by this encounter. At the end of the story, having previously claimed that the only pleasure in life was in "meanness," he now declares that violence and meanness is "no pleasure in life." Change is possible even in the most unrepentant of characters, this final conversation seems to suggest.
We learn that she is only selfless and nice when put in a situation that could kill her. Just like the Misfit says, "You'd be a good woman if you had a gun to your head everyday." Remember, it's the grandmotehr's fault that her family are dead.