I tended to see the grandmother as an overall selfish character. I understand the other perspective and can easily see how both would be entertained. Considering O'Connor's religious themes and background, it would seem logical that the grandmother in this moment receives grace in some sort of epiphany and that she finally becomes a good woman when facing her own death.  On the other hand, she may just be trying to save her own life. So, there's no real epiphany or grace here. What the Misfit says of her fits both interpretations. "She would have been a good woman if someone had been there to shoot her every minute of her life." This implies that 1) for a brief, and all important last minute, she was good and 2) she was never good until that last minute. Which begs the question, does waiting until the last minute count?  Is late better than never?

In the context of O'Connor's story, "A Good Man is Hard to Find," grace is something often undeserved, a force outside a character that generates an epiphany.  The grandmother has such an epiphany and receives grace as suddenly looks at the Misfit with a new perspective, seeing him as like unto herself.  At this point, the grandmother becomes a good Christian as she reaches out to the Misfit.  After he shoots her, the Misfit does recognize her transformation, for he says,

'She would have been a good woman...if it had been somebody there to shoot her every minute of her life.'

Significantly, after being shot the religious number of three times,  she falls over her legs which are crossed under her, symbolically like the crucified Christ, who died to save others.  For, after turning down the road to error, the grandmother redeems herself from the petty, materialistic life which she has been living.