Direct characterization in the novel, A Good Man is hard to Find, is presented in two ways including an analysis of the characters by the author, as well as, naming. No human is perfect, but most good people have decent morals and a conscience while evil people have no guilt or conscience; just like the grandmother and The Misfit.
Possibly the most famous line in all of O'Connor's work is The Misfit's observation, "She would have been a good woman […] if it had been somebody there to shoot her every minute of her life. When you can assume that your audience holds the same beliefs you do, you can relax a little and use more normal ways of talking to it; when you have to assume that it does not, then you have to make your vision apparent by shock to the hard of hearing you shout, and for the blind you draw large and startling figures.
It could be that it means a lot more than Sammy or the grandmother think it does. They stop for lunch at Red Sammy's Famous Barbecue, and the grandmother and Red Sammy commiserate that the world is changing and "a good man is hard to find.
O'Connor had a high opinion of Dante Alighieri's writings, especially The Divine Comedy, and she could not have overlooked the aptness of the line, "As many coals produce a single heat.
The misfit, on the other hand, is the character that seems to lack moral conscience and is constantly in conviction. He is the author of Rahnuk.
However you look at it, the grandmother appears to use the word flippantly. Christianity is either true for everybody or not true for anybody.