The answer to this question can be found in Chapter 22, when Sherburn, after killing the poor drunken Boggs for insulting him, is actually used by Twain to voice his feelings about the cowardice that lies at the heart of humanity. Note what he says to the mob that have been whipped up to go and lynch him for his crime as he faces them down with a gun:
The idea of you lynching anybody! It's amusing. The idea of you thinking you had pluck enough to lynch a man! Because you're brave enough to tar and feather poor friendless cast-out women that come along here, did that make you think you had grit enough to lay your hands on a man? Why a man's safe in the hands of ten thousand of your kind--as long as it's daytime and you're not behind him.
Sherburn is used by Twain to harshly criticise the moral cowardice that lies at the heart of so many men. A huge crowd may be easily whipped up into a frenzy, but that does not make them men. Sherburn, with the ease in which he faces them down, clearly displays that they are not "men" and are just cowards, and makes a harshly critical point when he talks about how so many mobs pick on those who are defenceless anyway, such as solitary women. Just because they can lynch a woman does not make them a man. The fact that Twain uses Sherburn, who has shown himself to be a callous killer, to give these words makes his message all the more forceful.