The king is the older of the two con artists that Jim and Huck meet in Chapter 19, and he’s definitely the worse of the two. Twain points out this fact in the first con the two men pull; the duke cons a harmless nine bucks while the king steals over $80 under religious pretenses by taking advantage of people’s religious faith.
When it comes to conning the inheritance money of Peter Wilks, notice that the king takes the active role while the duke acts the part of a deaf-mute. (This accentuates the fact that, all along their adventures, the duke has been figuratively deaf and dumb to the immorality the king convinces him to partake in.) When the duke has a moral crisis, the king talks him through it and back down the generally despicable, immoral path. If we could only tar and feather one of them, we’d pick the king. Although, as it turns out, such a decision is never necessary. Justice is served, you might say, when both men meet their sticky end.
Both the king and the duke are important characters in Huck’s moral growth. As Huck observes them, he is able to judge that what they do is wrong, and learn accordingly by negative example. The influence of the king over the duke – the king’s ability to convince the duke to do bad things – raises some interesting questions as to how much power one man has over another. Huck, for example, was able to overcome the negative influence of his father. What about his relationship to Tom? His ability to befriend Jim despite the environment and influences around him?