Huck's sense of morality is developing throughout the novel. He isn't always sure as to what's the most moral thing to do, but he constantly thinks about it and turns the issue over and over in his head. And his position isn't light; he doesn't have anyone in his nearest family who would be his role model. So, he basically has to do it all alone. On the other hand, the Duke and the King are two swindlers who are everything that Huck himself could grow up to be if he weren't guided by his inner moral compass. In this way, Huck is both an inverted younger version of them, and their complete opposite. Through them, he is going to learn what he shouldn't aspire to; but also, they teach him (unintentionally, of course) what he could become.
