In "A Good Man Is Hard to Find," is the Misfit archetypal of the Devil?
I don't see the Misfit as archetypal; for something to be archetypal, it must be a model, a prototype of sorts. Because, at times, the grandmother's talk seems to be "getting to" the Misfit, I don't see him as purely evil. For brief moments, he seems to be taking in what the grandmother is saying. He doesn't entirely dismiss it. I see the Misfit as more of a symbol, actually. He does represent a form of evil, yes, but he also represents how even the worst people can have vulnerabilities that some people can dig into or tap into. He is evil, no doubt, but there is more to the Misfit than most think.
Personally, I do see him as archetypal. "Symbol" is too broad a term to attach to a character who parallels the Devil so closely. He's not just representative of some vague idea of evil but rather plays out the fall of Satan with too much matching detail to ignore. However, just remember this is all personal speculation.
1) He states his "Daddy" singled him out from the rest of his brothers and sisters as being the one who will not simply conform, but will ask questions--and difficult ones, at that. Compare to Satan being cast out of heaven for doubting the absolute supremacy of God.
2) He is sent to the penitentiary and can't remember his sin or, more importantly, a suitable justification. The penitentiary is described as being nothing less than hellish because of its emptiness. Since he's unable to remember having committed a crime that would deserve such hell, he states that he commits horrible crimes after his release in order to deserve it. It's a weird, disgusting logic, but it certainly fits the root of all evil.
3) When he first mentions his "Daddy," he says he has a heart of gold. Later, he talks about how he had a way of handling the Authorities. The Authority, in this case, would be humanity; although it cannot dominate his father, it can judge and either reject or take.
4) As the grandmother escalates in her witness of Jesus in attempting to convert the Misfit, he becomes more bitter. Ultimately, he says that Jesus "shown everything off balance" when he resurrected the dead. The Misfit, then, leads the living astray in order to tempt them and take their lives again.
Biblically, Satan is also an implicitly essential balance. Jesus is the right hand of God, and Lucifer is the left.
5) When the Misfit talks about his past careers, he seems almost omnipresent. First, he says he was once a gospel singer (a state of grace), then an undertaker (grotesque); he saw a woman burned alive (witches) and plowed the Mother Earth (general presence).
6) One of his henchmen, wearing stolen clothes, has a silver stallion emblazoned on his chest. Horsemen of the Apocalypse? (This one is a pretty long stretch. There are only three henchmen and there are four horsemen, and, but for the stallion, there is no physical horse. Still, it's worth mentioning.)
There are other similarities, but this should get you started on the right track. I repeat, though, that this is just personal speculation.
The following is a much more balanced resource that, near the end, shows that O'Connor truly didn't mean for the Misfit to be a total embodiment of the devil. The differences between her story and the Biblical comparison are what make up her true message, which I haven't even touched. I don't claim credit to it.