The novel is a continuation of themes of isolation and group dynamics from King's previous novels and novellas, particularly The Mist; also, like Albert Camus' The Plague there is a strong element of Nazi allegory (Camus' novel describes the Nazi occuaption of France as a bubonic plague outbreak leading to total quarantine, as well as being a rich and powerful disease-horror novel in its own right.) King signposts both of these aspects of the novel rather obviously through conversations, and the parallels with the Third Reich are also very direct - the similarity between the Hitler Youth and the Dome's teenage police recruits, and Big Jim's eventual confinement to an underground bunker are two examples.
The Mist was mostly about how regular people can make errors, and find themselves driven to terrible acts due to isolation, stress and ignorance. The Dome complements this by showing just how evil and influential people can control such a population and turn the situation to their own ends - and this time the issue is not a lack of information but the inability of external forces to intervene. The Dome has appeared around a town where the leaders and some other individuals and groups are already corrupt and twisted - and they quickly act to consolidate their hold on power. It's not a subtle plot, and this is probably not a book for young children: hot-tempered and amoral youngsters are drafted onto the police force and quickly learn to abuse their power, getting away with brutality, rape and murder, while a series of accidental and deliberate acts leads to disaster upon disaster for the dwindling population.
Apart from the network of cynics who are already running the town, most characters start out neutral but quickly turn to either good or evil. In a situation like this King is probably correct to describe the group polarizing to extremes, taking on the roles of guard or prisoner. Rather than allow human ingenuity to defeat both the barrier and the fascist-like evil that has taken hold, the heroic characters barely manage to cling on to life, and the ending is not a triumph but a War Of The Worlds humbling.