To unpack the meaning of this novel, perhaps consider Rosamund as a protagonist. In this case, Jamie is something like her anima, because Jamie is mysterious to her, but promising, and after all, Jamie is scary to her, but that's because he already knows all the vital information it takes to survive in this backwoods town. And, he used that knowledge to save Clement, whose name is religious, so perhaps Rosamund is looking for the hero among the underworld who can help her.
This makes the story similar in kind to Persephone's story. The point mythically would be something like this: If you ever find yourself doomed to hell, or to Mississippi, being with the top dog is a helpful trick. There is good reason to consider these stories for comparison and contrast, because Jamie clearly saves this family from harm, with the implication (in his being a literal robber) that he doesn't help just anyone. Why is family safe? Because Jamie likes Rosamund.
The foils of the novel provide a kind of "double helix" effect to the story, like Big Harp and Little Harp, who illustrate the duo bad-guy motif, leading the reader to consider this gnarly idea: Isn't the implication in the denouement that Jamie and Rosamund will be a criminal duo, just like the Harp gang? And again, the foils of Salome and Clement show something about Jamie's relationship to Rosamund. In literary criticism, one might notice that the story is a chiasmus in form. The plot is true gothic genius.
