After Huck admits the truth to Mary Jane in Chapter 29 of "The Adventures of Huckleberry Finn" that the king and duke are imposters posing to rob the Wilks family of their inheritance, there also comes the moment of truth for the duke and the king in Chapter 30. When the king attacks Huck for "tryin' to give us the slip," the duke comes to his defense, identifying with Huck as one fleeing from a mob,
'Leggo the boy, you old idiot! Would you a done any different? Did you inquire around for him, when you got loose? I don't remember it.'
Subsequently, they argue about who has hidden the money in the coffin,the duke admits that he had thought of taking the money and giving everyone the "slip," the king, too, confesses that he was also so motivated, "'Nought!--I own up!"
Thus, for the first time, the king and the duke display some honesty, disproving the old adage that "There is no honor among thieves." However, after they retreat to take comfort from "their bottles," another adage does prove true. For Huck narrates that "they was as thick as thieves" again.
In Chapter 30, the duke and king have managed to escape the crowd and meet up with Huck and Jim on the river again. The duke and king argue over who hid the money in the coffin, both swearing that they didn't do it, but eventually owning up to the fact that maybe they had each thought of taking the money themselves, but that someone else had done it before the duke or king could act on their desire. The two men are mean-spirited for sure, but they also know that in order to work together they have to have some trust in each other. The fact that the duke and king admitted to each other that they had each thought of taking the money signifies that somehow, there is a shred of honesty in them. And maybe even a sort of friendship. They are both very similar people, and I think they really understand each other, even though they are both nasty people!
Both admit (honestly) that they had thought about stealing the gold back from the orphans, but had not done it. The fact of their honesty is one point against their villainous natures. However, the greatest admissions come from the duke near the end of the chapter, where he curses the king for "wanting to gobble everything." He goes on to tell him that he should be ashamed for standing by and letting it "be saddled on to a lot of poor niggers." Essentially, he should feel guilty for letting the slaves take the blame while they stood around & did nothing.
Of course, this is tainted by the fact that they willingly sold other slaves, splitting up families, and basically attempting to bleed the orphans girls of everything they had. So the question of their villainry has to be weighed against each action.