The story begins on a desolate farm in the middle of nowhere. It is a rural setting with a woman and her daughter sitting on a porch. From this opening, the tone feels quite relaxed, a vignette of the simple, country life but this is complicated just a bit by the word "desolate."
Anyone familiar with O'Connor's work might assume that religious undertones and violence will come into play, so that could make for an anxious tone to the reader. Even one unfamiliar with O'Connor will note the religious hints early in the story. Mr. Shiftlet "swung both his whole and his short arm up slowly so that they indicated an expanse of sky and his figure formed a crooked cross." He also reveals that he is a carpenter: both clear references to Jesus Christ. The tone becomes religious. And since this story has the words "life" and "save" in it, the tone seems to indicate a morality tale of potential redemption or a fall from grace.
Mr. Shiftlet appears as a Christ figure, purposefully so. He claims he simply wants to help around the farm in exchange for food an shelter. He even speaks in broad religious maxims such as noting the difference between the body and the soul and that men truly do not know what a heart is. Of course, the tone shifts when Shiftlet abandons the daughter and leaves, stealing the car. He is depressed, picks up a hitchhiker, who symbolically tells him to "go to the devil" to which Shiftlet drives on and offers a prayer to God. Paralleling the shift in tone from redemption to perdition is this "shifting" from praying to sin, sin to praying.
The story's main shift in tone is from a tale about Christian goodness (hopeful) to human frailty (dark and depressing). The old woman buys her independence, essentially selling (by paying) her daughter. Shiftlet presents himself as a caring, Christ-like figure but ends up revealing himself as a flawed man. There is another spark when the young man erupts on Shiftlet and jumps out of the car. It is a (verbally) violent outburst, perhaps meant to shake Shiftlet to his senses.