Much like Whitman's "Wild Frank's Return," this story is the semi-true tale of bad ends. In this case, a crooked lawyer gets himself in charge of the property of a young brother and sister; and he tries to marry the sister (to more firmly get the property), only to end up insulting her so badly, that the brother (drunkenly) kills the lawyer. And he gets away with it! So far, so true crime.
But Walt's story goes beyond true crime, into the realm of criminal philosophy. Er, I guess I mean "ethics"--though how much more popular a subject would it be if it was called "criminal philosophy"? See, the murder was witnessed by only one person. This person decides not to come forward when he sees the remorse and horror on the murderer's face.
In fact, thanks to Walt's all-seeing, semi-Transcendental eye, he tells us that the murderer goes through such a hell of remorse afterwards, that the death penalty hardly seems necessary:
O, if those who clamor so much for the halter and the scaffold to punish crime, could have seen that sight, they might have learn’d a lesson then!But he recovers and realizes that the world is still good. In his earlier versions of the story, Walt went on to record how the murderer atones by taking care of the sick, even sacrificing himself to heal the children of the original crooked lawyer. Still, even if this later version pulls back on that melodramatic end, there's some gestures towards that end, as in the above quote: maybe our notion of justice isn't perfect. Maybe "one wicked impulse" does not make a man worthless or totally dangerous.
From a craft perspective, this story starts in a way that seems very 19th-century to me: with a long paragraph introducing the antagonist--and introducing him first by mentioning the wider geography of the city. How many modern stories start with a street in the first line and only mention the protagonist (the murderer and son) in the second paragraph.
Or is the son/murderer the protagonist? In some ways, the question the story (literally) asks--the position that the story puts us in--is about the witness and his actions. We hear little of that character--only that he belongs to a "scorn'd race" (which probably means black, but not necessarily). That somewhat interrupts most readers' identification back in the day, but still: whereas the murderer is driven by impulse and the lawyer by evil and greed, only the witness makes a clear choice.