If all you are asking here is what "didactic" means, the meaning of this word is something like "meant to teach a lesson."
So, what this quote is saying is that this novel is meant to teach its readers a lesson. The quote is saying that this novel, like the other ones mentioned, has a good guy and a bad guy among the character. The reader is meant to clearly understand who is good and who is bad and to learn a moral lesson from the conduct of the two characters.
This can be contrasted to a novel where all of the characters are sort of "gray" in terms of their morality. Those kinds of novels are meant to make us think, not to teach us a specific moral lesson like these didactic novels are.