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.