I don't think there is an actual moral to this story (or lesson). The main point of the story, in many critics' opinions, is in the revelation of the theme (redemption and salvation) and how people can find salvation and redemption in the most unlikely of places and how those places aren't always good ones or ideal ones, either, as in this story. Unfortunately, the grandmother's revelation comes to late to save her own life, although one could argue that at least she HAD a revelation.
To understand the main theme of the novel one has to understand the conflict between Catholicism and Christianity in how they interpreted purgatory or atoneing for sins. Catholics believed that our actions would come with a price tag while Christians, particularily Southern Baptist believe that Christ forgives people for sins as long as they truly seek forgiveness and are geniunely open to receiving the goodness of Christ.
O'Connor points out the weaknesses to this position in respect to how the grandmother (southers) were preoccupied with keeping up appearances and this was no better revealed at the end of the novel when the Misfit says "She would've been a good women if she had someone want to shoot her everyday of her life". Suggesting that she was only acting civil and outwardly friendly because she feared for her life and said whatever was necessary in order to stay alive.
O'Connor cleverly uses irony to illistrate another weaknesses of the Christian argument regarding salvation in the afterlife. According to Southern Baptism, since the Grandmother was killed by the Misfit she would in essence be awarded enterance into heaven according to Christians. Therefore, O'Connor used the Misfit as the Grandmothers savior to parody Christian dogmas inherent weakness of being accountable for our sins because he allowed the grandmother enterance into heaven by killing her. That was the reason why she had refered to him as "her son" on more than one occassion.
I found the story intense but extremely interesting in how beliefs are formed from people we trust in our lives. The social phonoma that occurs in society from these beliefs, be it from religion or family. We come into the world as good children but, misguided and abuse socially as we grow up until frustrations from these, early learned beliefs, changes us into misfits as adults. The "good man" is hidden under all the lifes frustrations and the recovery to be a good men/women is difficult, too many traumatic events, so most forget what its like to be a good person.
I love this story and I like the way readers do not symapthize with the grandmother at all; after all, she is the cause of all the trouble--and death. The grandmother is a woman who is set in her southern ways, and these ways are a cultural metaphor for racism in the American South. The grandmother is a symbolic cause for the death of the South by ignorance. The Misfit is manipulated innocence; her does not understand his position. Damn, the story is intense.
Comparing O'Connor's "Revelations" to "Good Man.." really helps understand her intent in exposing the Southern gentlemen and women as hypocrites of Sunday practices and racism. How can a Southerner in taht time be some damn religious and some damn ignorant and hateful. Fkannery ubderstood this.