Whether or not the Misfit seeks justice is questionable. He kills because he figures that he was unjustly punished, so he may as well be mean since he has already served time in prison for the crimes. Things are in this disorder because of Jesus. He explains that Jesus threw everything "off balance" when He died and was raised from the dead:
If He did what He said then it's nothing for you to do but throw away everything and follow him, and if He didn't, then it's nothing for you to do but enjoy the few minutes you got left the best way you can--by killing somebody or burning down his house or doing some other meanness to him. No pleasure but meanness, ...
The Misfit is the grotesque of this Southern Gothic story by Flannery O'Connor. It is his depravity that effects the grandmother's salvation as he realizes that he is one of her "own children." By recognizing that she, too, is a sinner, the grandmother is saved when she, like Christ, becomes the sacrificial victim as she dies with her legs crossed, dying for evil as Christ died for the sins of mankind. In a grosteque fashion, the Misfit is the agent of grace, not one who seeks justice.