In "A Good Man is Hard to Find," when the grandmother's head clears for an instant, what does she suddenly understand?

The grandmother has a moment of what the Catholic Flannery O'Connor would have called grace. As she listens to the Misfit's weeping and his description of his spiritual anguish, she feels compassion for him and realizes the unity of the human family, whereas before she was superficial and snobby, looking down on others outside her race and class as inferior.
She tells the Misfit he is one of her own children, which is her way of saying the two of them are not so different as she previously believed, even though he is a criminal, and she is a lady. In this moment of grace, the grandmother comes into spiritual enlightenment; however, her attempt to touch the Misfit causes him to shoot her to death.