The motif of Hades, the underworld
Social rules don't apply in this novel. That's the general vibe of this small Mississippi town. For instance, Clement probably came into town nervous, but he treats his suspicions as regular old anxiety, and he tries to ignore the red flags. Then, on the same day, he learns from a literal robber (oh boy) that there was a conspiracy among the town to murder him for fun. This is one of many, many artistic details that paint a specific portrait: Hades. Clement is in hell.
The man of mystery
This novel is titled The Robber Bridegroom because of the archetypal depiction of Jamie. He represents a kind of man that Rosamund finds extremely enticing, and the rest of the story is designed to illustrate her gravitational pull toward her underworld savior, a bad guy doing the right thing for her. The mysteriousness of Jamie allows him to court Rosamund without subjecting her to danger.
The good side of evil
Jamie is a thief who has probably killed people. He hangs out with a rough crowd. He has enemies, likely, and if this were done openly, there's a chance Rosamund would be executed by a rival gang. That is precisely why she needs someone who understands the real situation, someone with sins under their belt, enough to understand the inherent risks of living in a place of great danger. Clement can't really protect her, because he's not an insider. He's actually liable to become a victim.
Clement the weak good guy
Clement is a foil for Jamie, because where Jamie has experience, Clement has innocence. When he learns a plan was made on his life, he doesn't respond the way Jamie might. As a career criminal, Jamie would put a stop to that, or escape and murder someone in the night; Clement just accepts charity from strangers and stays holed up and paranoid. He's an honest, good guy, but he is unfit for his environment, and his strict morals make him poorly adaptive.
The motif of bad-guy duos
Look at the Harp gang. Instead of having a kingpin, the gang has two brothers, a big one and a little one. This is a classic motif. In everything from Batman to Shakespeare, the idea of dual characters (on both sides) is very common. In this case, Clement and Salome are a pretty bad couple for this town, but Big Harp and Little Harp make one heck of a team. Then there is the novel's main story, the formation of a "partners in crime" relationship to survive a gnarly Southern underworld.
