At the end of "A Good Man is Hard to Find," the grandmother undergoes, in her terror and panic, a profound epiphany as to the shared humanity between herself and her murderer, reaching out to the Misfit even as he stands to kill her. This moment represents the moment of grace around which this entire story is structured: the grandmother, who had previously been characterized as selfish and judgmental, caring only for appearances and social pretenses, now experiences real empathy for another human being (and for her victimizer, no less). The Misfit's response represents a dark and cynical recognition of her change in character, on the part of the person who she was reaching out to (and who killed her).

It's an interesting line, when viewed from the perspective of the Misfit, who witnessed the grandmother's epiphany and whose own characterization displays the most extreme form of moral nihilism. As far as the Misfit is concerned, the universe and morality can be viewed along a binary: either Christianity is true (in which case, morality is true) or Christianity is false (in which case, there is no morality whatsoever). It is this binary vision, tying to religion and morality, that shapes his entire life of murder and crime.

However, at the same time, it is important to note that this binary also causes significant mental turmoil to the Misfit himself: he genuinely seems to struggle with this question and with the problem of God's potential existence, and he seems tormented by the fact that a clear answer does not exist. Seen from his perspective, the grandmother's change of heart, at the very end of her life, does seem to effect him as well: it provides him a glimpse into the kind of unconditional love which, Christianity holds, God represents. Seen from that perspective, underneath the cruel tenor of his words, his association of the grandmother as having had the potential of being a "good woman" seems to reveal a potential weakness (or weakening) within his own sadistic nihilism, which should not have allowed any such recognition at all.

The Misfit says that the grandmother would have been a good woman if she had had someone there to shoot her every minute because he feels abject disgust towards her. In the context of the Misfit's intensely negative feelings, his statement about the grandmother can be understood in two ways:

Firstly, the Misfit might mean that the grandmother would have been a good woman if she had lived every minute with her life under threat. Someone there to shoot her every minute might have given her reason to pause in her normal behaviors; if her life was in danger every minute of her existence, she may have have been more careful about how she spoke and to whom. Because none of her protestations had any effect at all on the Misfit, and in fact, they seemed to exacerbate his antipathy, the Misfit may have stated his dislike of her and her comments in a particularly violent way.

Another, darker reading of this statement by the Misfit suggests that he thinks that the the grandmother could have been a good woman only if she was dead. If this interpretation is true, it reveals a violent misogyny, or hatred of women, in the Misfit. "Someone to shoot the grandmother every minute of her life" might be another way of saying someone to ensure she is never alive, not even for a minute.

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.