This phrase comes from the short story, "A good man is hard to find" written by Flannery O'Connor.
This tale was published in 1953 and accompanies a vacation trip of a grandmother with her family. The old lady would like the trip to be another destination and unsuccessfully try to persuade the family to go to Tennessee rather than Florida. From the beginning, it is clear that the grandmother is a difficult and stubborn person, and that her relatives only tolerate her out of necessity because she does not show kindness in her actions.
The Misfit speaks the phrase "She would have been a good woman, if she had been somebody there to shoot her every minute of her life." to describe that grandma's goodness is a false thing, a seeming thing that only comes when the old lady feels her actions are being watched by someone and this one will judge what she does as good or bad.