What is the meaning of the Misfit’s comment that the grandma “would have been a good woman if it had been somebody there to shoot her every minute of her life”?

Towards the end of the story, just before she's shot by The Misfit, the grandmother experiences a moment of grace (a recurring theme in the work of Flannery O'Connor). She recognizes that The Misfit is a human being, like one of her own children. Prior to this moment, she's lived a life marred by moral hypocrisy and a chronically judgmental attitude towards others, including her family. But now she shows true compassion and understanding for probably the first time in her long life.
The Misfit, however, provides a much-needed sense of perspective. His famous statement contains two important meanings. First of all:

“She would have been a good woman . . . ”

The implication is that the grandmother was never truly a good woman. The Misfit may be a psychotic killer but he has a remarkable degree of insight into the grandmother's true character. Then we have the rest of the statement:

“ . . . if it had been somebody there to shoot her every minute of her life.” 

The grandmother only started to act like a good woman in her final moments on earth with a gun pointing at her. If only she'd lived like that every day of her life up until then she would've been genuinely good. And, perhaps, then her life wouldn't have ended the way that it did.