At the moment that the Misfit's face twists close to hers as though he were going to cry, the grandmother murmurs,
"Why you're one of my babies. You're one of my own children!"
It is at this moment that the grandmother is redeemed, for she recognizes her own depravity and sin in the spiritually grotesque Misfit. This black character then reacts by shooting her the spiritually three times through the chest. As he orders her to be taken off, he says,
"She would have been a good woman...if it had been somebody there to shoot her every minute of her life."
because he realizes that the grandmother's salvation requires an extreme situation since "Jesus thre things off." While the title of O'Connor's story supports the satiric side of the author, the use of a depraved man is what is required before the grandmother recognizes her own sins. Receiving grace in her martyrdom, the grandmother is shot the religious number and she collapses with her legs crossed--on the dark side of the cross where the experience of grace is violent, not sentimental.
Flannery O'Connor's extreme use of violence as a catalyst for a greater vision of spiritual reality is illustrated in her story, "A Good Man is Hard to Find." Critic Patrick Galloway writes that according to this philosophy,
the person in a violent situation reveals those aspects of his character that he will taken with him into eternity; hence the reader should approach the story by looking to such mmoments as an oppotunity to peer into the soul of the character.
Such, indeed, is the case with the grandmother of the short story, "A Good Man is Hard to Find."
The misfit says that the grandmother needed to have a gun pointed at her head every day of her life, because that's the only thing that could have convinced her that she wasn't God's gift to the planet, in so many words.
The grandmother is self-righteous and bigoted and unaccepting. She thinks she is better than everyone else and lets them know it.
Only violence can show her that she is not "all that," as we might say today. If you interpret her final words--her acceptance of the Misfit as one of her children--as a genuine epiphany, then it is violence that brings her to it. Nothing else could have changed her ways of thinking. In typical O'Connor fashion, the story reveals God's grace working in the territory of the devil: working in the territory of violence.
The Misfit is, apparently, an astute judge of character. The statement you ask about is perhaps proof that the grandmother's epiphany is genuine, and is recognized by the Misfit. Or, perhaps, her last words are just a ploy to convince the Misfit not to kill her, and the Misfit is being facetious. Either way, he has an accurate grasp of the grandmother's character.