In Flannery O'Connor's shocking short story, "A Good Man is Hard to Find," I believe the Misfit is symbolic, based upon the theme that seemed to drive much of O'Connor's literature.
Religion is perhaps the most important element of O'Connor's writing. The author believed that someone generally needed to be hit on the head with a two-by-four before he or she would notice that God was trying to speak with him or her.
In this case, it is the Grandmother who is not listening.
The Grandmother is extremely annoying. One might feel she drives the Misfit to kill her, except that he kills the rest of the family as well. She has no true sense of a meaningful, honest relationship with God.
The violence at the hands of the Misfit is what really wakes the Grandmother up to her own lack of faith. Even though her family has already been murdered, it is not until her own life is threatened that the Grandmother has a religious "epiphany" (awakening).
Symbolically, the Misfit may represent all that is hurtful, harmful or even evil in this world. In light of how O'Connor sees human beings and their faith in God—or lack thereof, it would seem that the Misfit represents the adversity and true suffering that comes knocking on one's door: often times, perhaps as O'Connor sees it, the only time someone finally has time for God—when things get really bad...even at the moment just before death.