In "A Good Man is Hard to Find," why does O'Connor present us with such an unflattering portrait of the grandmother?

O'Connor
presents a deeply unflattering portrait of the grandmother in order to give her revelation and
redemptive act at the end of the story even greater gravity and meaning.

In
the grandmother, O'Connor creates a character that we sympathize with but do not necessarily
"like." She is racially bigoted and pretentious. She is nagging and annoying, and her
family barely tolerates her. When she meets the Misfit, her initial response is to pathetically
beg for her life and then try to flatter him. She tells him that she knows he wouldn't hurt an
old lady, that he doesn't look "the least bit common," that he must come from
"good people." At the same time, it is impossible not to sympathize with her
predicament: she is a deeply flawed but authentically human person who finds herself in a dire
situation in which the only way out seems to be death.

The juxtaposition of
the Misfit's character with the grandmother's is, of course, central to the story. The Misfit
steals and murders, but is honest, reflective, and non-hypocritical.  The grandmother is
ostensibly a "good" person, but she lies to her family, holds herself above others,
and for most of the story is not even a little self-aware. It would be tempting to morally
equate the grandmother with the Misfit, and to say that she is just as "bad" as he is,
albeit for different reasons. However, to do this would be to miss the point of the
story. 

At length, it becomes clear that the Misfit is not going to let the
grandmother live, despite her most earnest pleas and entreaties. Suddenly, the grandmother's
"head [clears] for an instant" and she finally recognizes the Misfit's humanity. They
both belong to the same human family, and if not for the accident of circumstance, he might well
have been "one of [her] own children." The notions of faith and spirituality that she
had been merely babbling about while begging for her life now take on an authentic meaning. She
and the Misfit are united not by some sense of shared "evil," but by a shared
humanity, and she understands the moral obligation she has to the man who is about to take her
life. Thus, even though she does not begin the story this way, the grandmother dies a good
person - a beatific smile on her face just above the bullet holes the Misfit puts in her
chest.