Why does Mary Grace attack Mrs. Turpin in "Revelation"?

This is a very profound moment in the short story,
Revealation, by Flannery O'Connor. The story takes place in a doctor's office and Mrs.
Turpin is a self-righteous woman who thinks pretty highly of herself and thanks Jesus
that he has not created her to be "a nigger" or "white trash." There are several other
people in the office, and one of them is a white trash woman and the other is a nice
woman. The nice woman is the mother of Mary Grace. Mary Grace is an ugly girl, covered
with acne. Mrs. Turpin has been musing to herself, criticizing all of the other people
in the office, and eventually she makes this comment
aloud:

"If
it's one thing I am ... it's grateful. When I think who all I could have been besides
myself and what all I got, a little of everything, and a good disposition besides, I
just feel like shouting, 'Thank you, Jesus, for making everything the way it is!' It
could have been
different!"

At that point,
Mary Grace freaks out and attacks Mrs. Turpin. Mrs. Turpin asks
her:

"What
have you got to say to
me?"

Mrs. Turpin's question,
I believe, is really a request from God to see what HE has to say to her. If you read
some background on Flannery O'Connor and read her other stories, you will find that the
concept of Christian "grace" is a very strong theme in her writings. So when Mrs. Turpin
asks this question, she is asking it of God, who is the only one that can give us grace
(unmerited favor).

Mary Grace's answer to Mrs. Turpin
is:

"Go back
to hell where you came from, you old wart hog!" says Mary
Grace

This is God's message
to Mrs. Turpin. She thinks she is clean, lily white, above everyone else, but God says
no, you are a sinner like everyone else, in need of grace. Mrs. Turpin things she is
heavenly, holier than everyone else, and God tells her no, you are going to hell like
everyone else unless you accept my grace.

Mrs. Turpin is
upset by Mary Grace's answer and for the rest of the story grapples with this answer,
really grappling with God's revelation to her, which she does not like, with good
reason. She is a hypocrite.