As in most of her stories, O'Connor shows the sinful side of human nature in her grotesque characters and violent plots. The unregenerated human soul is evil and that is the nature of man. Only God can redeem a human being from his sin nature. The grandmother is petty, annoying, and bigoted, but even she is offered grace by a loving God at the moment of death. O'Connor's characters often do not wake up until right before she kills them off, as with the grandmother, but in doing this, O'Connor shows that it is never too late for God to redeem a person, no matter how odious that person is. In contrast to the grandmother, the Misfit rejects redemption. He tells the grandmother that he is doing "just fine" on his own, which is really not true. He is not doing fine, and by rejecting Jesus by telling the grandmother that "Jesus turned everything around", he is not redeemed at the end, like the grandmother is. By making her characters obnoxious and often hateful, O'Connor attempts to show that even though human nature is evil, God loves humans anyway and is willing to offer them grace. They have to accept it, however, and not all of her characters do so -- as this story illustrates.
Read about the themes here on eNotes.