Sherburn expresses a clear disdain and lack of respect for the men in the lynch mob. His speech is notable for a number of reasons, including the lack of any apology for killing Boggs.
The lynch mob is ostensibly collected to see justice done. Sherburn, however, does not speak about justice, nor about his decision to shoot Boggs (after warning Boggs that this would happen if Boggs failed to stop harassing him). Instead, Sherburn speaks about cowardice.
Holding off the mob with a gun and a sharp tongue, Sherburn demonstrates his point rather thoroughly. Not only does he succeed in breaking up the crowd single-handedly, but he also tells the collection that they are less than men, cowards, and frauds.
The men presume to be brave because their culture has told them they are brave, Sherburn says. Yet this culture is characterized by cowardly behavior. For fear, they fail to convict killers. For fear, they act in large, masked groups at night instead of as individuals in the light of day.
Sherburn here is a tool for Twain's invective against false, collective morality.
Twain speaks out against lynch mobs who do not fight with courage but come like cowards in the middle of the night wearing masks.
This is Sherburn's clearly stated opinion. Additionally, we can see this address as part of the novel's theme, largely explored through Huck, regarding the invidual's relationship to society's morality or society's moral code. Sherburn articulates a preference for individual responsibility and personal morality over the morality of the crowd, wherein individuals defer responsibility to the group.