I would add the comment to that interpretation as to what O'Connor's tone is in presenting the Misfit's final observation. Does he speak the truth? After all, the title of the story is "a good man is hard to find," "man" referring, I think, to human being in a generic way. We are all deeply flawed, O'Connor suggests, and that is what we share as human beings: an original state of sin through which we must find redemption. The grandmother acknowledges her connection to him when she calls him one of "her babies," suggesting that she participates in the production of evil, that good and evil are not dialectical opposites but rather deeply related. So, his words offer wisdom on the events of the day, that he in some way does her a good deal by murdering her because in doing so he gives her the opportunity to be "good."