Both are stock characters, used as foils to reveal more clearly the traits of the protagonists, the grandmother and Rainsford. Both are mass murderers who give their prey opportunities to save themselves. Zaroff gives Rainsford more than a fighting chance, and there seems to be a gentlemanly respect between the two foes. Zaroff waits for his prey to wreck on his island, whereas the Misfit stumbles upon his wrecked victims.
Zaroff is an archetypal antagonist in escape literature (like other eccentric mad scientists), and the Misfit is one in a religious story, and he seems to have the moral high ground on the grandmother. In fact, O'Connor uses the Misfit as divine evil. The Misfit tells his accomplice to throw the grandmother into the woods with the others, adding:
"She would have bee a good woman if it had been somebody there to shoot her every minute of her life."
So says Thelma Shinn,
...[O'Connor's] criminals, her misfits and prophets, are closer to salvation because they are in the spiritual realm: they are Evil and are fighting a religious battle within themselves--their belief or disbelief in Christ is to them a matter of life and death.
She provides her secular grotesques higher spiritual ground at the expense of social moral good because she cares more about their perfervid spirituality (their souls) more than their nature (that they are evil).
Although Connell's story is great escape literature, there's more depth to O'Connor's and, thus, to the role of the Misfit.
Both General Zaroff, in the Richard Connell short story "The Most Dangerous Game," and The Misfit, in Flannery O'Connor's tale "A Good Man is Hard to Find," are serial killers who seem to enjoy their chosen livelihood, though Zaroff takes more pleasure in the actual hunt. Zaroff captures and hunts humans because they provide him a more elusive prey than the wild animals that no longer interest him. To Zaroff, the kill is a game, and the victims are his trophies.
Although The Misfit's motives are less obvious, he also seems to be adept at killing, a fact the grandmother has not overlooked.
"...you read here what it says he did to these people. Just you read it. I wouldn't take my children in any direction with a criminal like that aloose in it."
Like Zaroff, The Misfit is also polite and hospitable at first, but they are a different pair: Zaroff is a Cossack, known for their particularly violent ways, while The Misfit is "a different breed of dog." Unlike Zaroff, who was born to a wealthy family and enjoyed a life of privilege, The Misfit has always been dirt poor and has seen very little good in his lifetime. The question that The Misfit proposes to the grandmother just before he kills her seems to sum up the main difference between Zaroff and himself.
"Does it seem right to you, lady, that one is punished a heap and another ain't punished at all?"
Zaroff has used his military background and wealth to kill without consequence, while The Misfit kills because "I can't make out what all I done wrong fit what all I gone through in punishment." Zaroff kills because he enjoys what leads up to the kill, while The Misfit can't help his urges to kill.