One of the themes of the novel relates to the notion of "individual vs. society". This theme is often expressed through Huck in his moral considerations, as he tries to determine whether it is best to follow society's rules or follow his own sense of right and wrong.
In this chapter, however, Huck has almost no role in the action. He is merely a member of the audience looking on as Sherburn repels a mob out to lynch him.
The action of this chapter features one man chastising and deflating an angry mob, deriding the crowd. He tells them they are not men and they are led by a "half-man", Buck Harkness.
Sherburn's position is literary an example of one against many and his speech gives further articulation to this concept.
He has done what he felt was necessary in warning then killing the drunk man, Boggs. When the crowd rolls up to his doorstep, Sherburn does not apologize or excuse his behavior. Instead, he gives a moral lesson to the crowd, teaching them about courage and cowardice, clearly demonstrating that he is willing to act on his own conscience.
Implied in his speech is a commentary on the weakness of morality developed by masses, mobs, and frightened crowds. This weakness is explored in numerous other instances in the novel, as groups are defrauded with ease and shown to express poor collective moral judgment.