Jürgen Hubert said:
My point is that once the PCs become powerful enough to defeat whole armies by themselves, they should think in grander terms than just slaying bigger monsters.
That is not what the game is designed for.
Power and the use of power speaks to morality and ethics.
The morality of D&D – with in the novels and the published settings – is simple, even simpler than that of comics books. Evil and good, black and white, left and right. All in all, all an NPC has to do is call themselves something (“good” for example) for them to be that, even if none of their other attitudes, behavior, treatment of others and temperament actually match the criteria for the quality. This is true of PCs only if DMs allow it, though most players will spin and spindle any situation to justify any immoral act they can.
Ethics is more determined by profession and situation, i.e. business standards and table manners are more ethical matters than moral issues. In terms of D&D a wizard is ethical if he or she acts like a wizard and unethical if they act like a ranger. This is true of NPCs (usually) and PCs both.
That is what the system is designed for and all the system is designed for. It is not designed for the kind of moral or philosophical (or social or political) inquiries you are asking about. All the rulers (good and bad) are in place and questioning their role (or roll) is at best pointless.
None of the hack-and-slashers would support making these inquiries in a particular game (nor into the game as a whole), and even mentioning it is an invitation to have people shout you down.
Ultimately, the only thing the Rules As Written allow for is the PCs to be bloody minded grave robbers and scalp hunters living and dying at the whim of unapproachable kings and queens.
That is just the way things are.