In "A Good Man Is Hard to Find," O'Connor explores themes of redemption, hypocrisy, and the nature of evil. These themes are ultimately centered within O'Connor's use of characterization, in particular the degree to which the Misfit and the grandmother are employed as foils to one another.
Ultimately, the grandmother's own characterization is based in a fundamental contradiction: that the self-professed good and respectable Christian woman would so thoroughly transgress against the tenants of Christianity as described in the Gospels, with her vain and judgmental (not to mention racist) attitudes and behaviors. In this, O'Connor explores the everyday hypocrisy and pettiness that is common within the human condition, qualities that are practically embodied in the character of the grandmother herself.
In contrast to the grandmother, O'Connor creates the character of the Misfit, with the two serving as powerful foils to one another. Whereas the grandmother can be said to represent the more mundane side of human evil, the Misfit is a force of active malice and nihilistic cruelty in the world. Where the grandmother is a hypocrite, ignorant to her own shortcomings and the damage she might cause, the Misfit, on the other hand, is entirely self-aware and honest in a way that the grandmother (on account of her ignorance) cannot be. With that being said, the Misfit has a two-part role in this story, for he is simultaneously the grandmother's murderer but also an instrument for her redemption and (in Christian terms) salvation. After all, it is in this encounter with the Misfit, and in that sense of genuine empathy and human connection which she forges with her killer, that the grandmother actually embraces a Christian mindset, if only at the very end of her life.
These themes are further expressed and amplified through various other literary devices. For example, note O'Connor's use of foreshadowing, with perhaps the most notable example present in the story's opening paragraph, with its reference to the Misfit. In this respect, long before he is encountered, the Misfit maintains an invisible presence across the entire story, one which will culminate in his later appearance and the abrupt tonal shift that follows his introduction.