Oh I gotta expand on this (late I know, it lingered in my drafts) cause this gets at how they tried to use Bahamut as an excuse, when by its own verse, that’s not Charioce’s sole motivation.
First : even if it wasn’t temporarily, it still wouldn’t be worth it.
With the exception of stealing Dromos, his had nothing to do with any strategy for “permanently sealing Bahamut” (as that one episode description put it). The argument of necessary evil could not apply here, and it doesn’t even make for a cool motive because it doesn’t make sense.
The show briefly suggests that he is …
…. in need of funds for potential wars that he could avoid by telling people about Bahamut’s looming threat and …
… keeping Bahamut’s rift stable/closed.
Bahamut ruined the kingdom a decade ago, but if he can’t fix his economy without slavery, he’s an incompetent king. He launched a costly invasion into another realm where none of his soldiers had experience, two years after he’d already captured the tools to face Bahamut, rather than simply build up defenses against the demons and inform the gods that Bahamut’s coming back way sooner (if we’re assuming he isn’t the reason it’s coming back sooner) and occupy said gods with keeping it sealed till he’s done building Dromos.
His adviser even points out the gods are the better option after they observe their human workers barely manage to seal the rift :
He rejects it. He has the option of completely free of charge barrier maintenance and doesn’t use it, at the expense of his own people. This is not a rational choice, and it does not match the “pure gentle heart” we’re supposed to believe is the real him.
Likewise, the way he installed slavery doesn’t actually benefit the kingdom much either, it’s first geared at inducing suffering. The botched open wound amputations mean that a whole lot of otherwise healthy slaves die, not to be sold. The lack of wings means any advantage they have in construction is a moot point; the collars work at a distance (or Azazel would’ve just flown out of the arena) so there is no need for wing amputations to restrain anyone. This is not rational, it is cruel.
He does not use any demons to build Dromos, human slaves are used for that, so there isn’t even any relation between abducting and enslaving demons, and the economic excuse doesn’t hold because it’s not done economically at all.
The death fights in the arena kill off all the strongest demons or they are absorbed into the armies to be cannon fodder. That’s the closest use he got out of it, and even that is only marginally related to building Dromos in that it somewhat preserves troops/assuming he owns the arena it gets him some money. But again, there are far more constructive ways to use powerful demons, so this doesn’t work as economic excuse either.
In the end the show acknowledges that the winged and large demons would be useful for construction. Putting all other jobs aside, having the entire city constructed like his castle would surely help once he brings Bahamut in raining destruction. At least the welfare of his own people is supposed to matter to him, right?
They shouldn’t be excusing slavery with economy to begin with, but the way they did it doesn’t even hold water.
He’s annihilating the two tribes that kept Bahamut at bay ten years ago. Dromos has no inherent shields and only one shot. He shouldn’t be bringing Bahamut to the capital and he shouldn’t be weakening the only entities that could create massive shield (without turning to black blobs). He has had choices between cruelty and what is good for his kingdom, and yet chooses cruelty.
The choices Charioce made how slavery is handled in his kingdom are geared at weakening the demons tribe first and foremost. This makes no sense for defeating Bahamut, but in his own words, that’s not his only goal :
He wants the human tribe to come out on top, to continue to prosper in the future without fearing demons or bowing to the gods.
The way the disagreement between Kaisar and Charioce was phrased wasn’t really about good and evil, but about what methods would benefit the humans the most. Even in scenes when Kaisar doesn’t have to restrain his opinions, he just describes Charioce’s methods as too extreme, rather than his crimes being pointless altogether and utterly reprehensible. They consider whether coexistence was possible, not whether slavery and genocide is evil.
Charioce got exactly what he wanted in the end. No need for redemption before the narrative presents him a pure hero, it’s the victims who are pressed to change their opinion, who must learn their lesson, who we are to think were wrong for defying him.
No need for Charioce to have any kind of consequence either. Dromos vanishes before he woke up so he doesn’t even have to deal with blow back from his own people for taking away their slaves. The benefits of the new coexistence all fall to the humans, who now have a bunch of underpaid supernatural workers at their disposal who conveniently do not want to go to their homeland and rebuild that. The gods act as visitors rather than deities now, laying the foundation for unraveling the religion as well.
All Charioce had to do to keep reaping benefits off of his victims was realize that huh, maybe he doesn’t need to go quite that far to get what he wants. There’s this whole undercurrent that the god and demon tribes needed to be beaten low, even the innocent citizens, to stop being evil or arrogant.
They wouldn’t even change the painting to include the gods and demons that protected Anatae from Charioce’s theatrical decision to have the fight with Bahamut right on top of the capital. The victims should be satisfied with the barest decency after all their suffering, all the glory belongs to Charioce.
Presenting a real world evil such as slavery, genocide and dictatorship in the form of a “misunderstood” woobie who just went a little too extreme but ultimately made the world better by committing atrocities, while his victims rising and uniting as equals is demonized and had to recognize what a good guy their oppressor really is, that is at the root of what makes this story so offensive.