This hints at something terrible far in the distant future. Could this be Armageddon? If this were true, then the arrival of Karellen, the Devil, is the coming of the Antichrist. But if that's so, then Karellen seems to be an awfully friendly Antichrist. On the other hand, there are some people who might argue that a utopia is nothing less than Armageddon, because it would almost certainly result in the stagnation of the human race--a kind of intellectual Armageddon. But the most important thing is the irony: the Devil as the savior of humanity.
Chapter 6 deals more with the issues of a utopia itself. First, there is the question of how such a utopia is created. Karellen tells Stormgren that all political problems can be solved through "the correct use of force." Stormgren rightly points out that this seems to argue that "might makes right"--that is, the Overlords are just bullying humans into doing the "right" thing. But the important word is "correct." By using the "correct" use of force. The Overlords use subtle methods to make sure the humans do what they're asked to do. Karellen tries to minimize the amount of human suffering.
The utopia created by the Overlords certainly sounds appealing. Everyone has enough to eat, everyone can do whatever they want. There is little crime and disease, and no war or poverty. But there are already clouds on the horizon: there are no longer any good works of art, and soon, people may soon become bored. People in a utopia lack a sense of purpose; this may soon become a problem for many people on Earth.