I'm having trouble understanding why the Misfit is redeemed. Many may believe he isn't redeemed in the story, but I think he is. I'm just having trouble understanding why he would be redeemed in the first place.
The previous posts make a strong argument, I think, that the Misfit is not redeemed but is the vehicle of the grandmother's redemption.
The Misfit denies his past crime (killing his own father) and so moves through the world without remorse and therefore in an amoral state. He does not consider what he is doing to be right or wrong. He simply acts - violently.
This behavior is difficult to square with the notion of redemption.
In accord with the previous post, the Misfit is the "grotesque" who effects the grandmother's redemption. In so many of O'Connor's stories, grace often accompanies a moment of violence, and the use of the grotesque, critic Gilbert H. Muller states, is employed by O'Connor "to reveal underlying and essentially theological concepts." O'Connor herself commented that violence returns characters to reality and prepares them to accept their moment of grace. The Misfit acts as the vehicle for the grandmother's moment of grace.
I don't actually agree he is redeemed. Whilst the grandmother is definitely redeemed through her epiphany that the Misfit is another one of her "children," the Misfit shows, that although he is potentially close to redemption, that he wishes to reject it, thinking that he is beyond the ability to be redeemed. This is confirmed by his act of killing the grandmother at the end of the tale.