What are Parker's appeal in terms of both versions of Aristotle's theory of rhetorical ethos. That is, you need to analyze the attributes of this character that allow audiences to find traces of "good character" in this otherwise completely unappealing sort of person. When it comes to the second version of Aristotle's theory, you'll need to profile the audience for this kind of fiction.
Who reads this sort of novel? Men or Women?
Readers in their teens? Their 20s? Their 30s? Readers even older than that? What is their socio-economic status? Are they rich or poor or middle-class? Are these readers college-educated or not?
Etc., Etc. I already take it for granted that all sorts of people might pick up and enjoy this novel. What you need to profile is the novel's "target audience." Richard Stark wrote this book for a very specific kind of reader, and the novel was a success because this specific audience showed up in droves to buy it.
It made a fortune, and it led Stark to write about a dozen or so sequels - all of them money-makers because they appealed to one particular audience.
What are the fears and daily anxieties experienced by this audience? What do they fantasize about? What do they despise?
What are their values? How exactly does Parker appeal to these readers in light of their particular fears, fantasies, hatreds, and values?