Sherburn is explicitly disrespectful of the men attempting to lynch him. He holds them off with a single shotgun and a speech, deriding their moral fiber, their manhood, and their violent whimsy. He goes so far as to say that there are no real men in the mob and only one "half-man" to lead them.
Sherburn's disdain is clear as he outlines the ways that the people of this culture have been falsely led to believe in their own bravery. He also does not apologize for killing Boggs. His complaint against Boggs is not unlike his complaint against the lynch mob. A lack of accountability mars the character of the men of this culture. They prefer to hide behind masks and behind one another instead of acting boldly.
They are, in essence, cowards to behave as they do.
Twain speaks out against lynch mobs who do not fight with courage but come like cowards in the middle of the night wearing masks.