You're confused. Yes you are right the writing isn't great and he could make his societies better and his subjects will be unlikely to revolt if he drowns them in comforts. But he's simply being a King, not a warlord.
Dwight's not trying to rule "human society". There is no analogue to feudalism here. It's impossible to "fix" this multi-species and monster society non-violently because he is imposing an Order that is antithetical to all living beings involved.
He's forcing different species to live together in a single non-segregated society. Some of which are inherently violent, illogical, and stupid. They have self-destructive biological impulses for violence even when comfortable. Predators and Prey living in peace like it's Zootopia. That unity is eternally fragile and requires tremendous authoritarianism with, eventually, generations of brainwashing the children to not see any differences.
The only successful time this happened in history is Singapore, which required continuous police monitoring paired with brutal and instant punishment to enforce peace and stability.
...? I'm not confused?
I did not say at any point in my comment that Dwight's ultimate goal is to rule human society. I merely said that if his goal is to "put a stop to human-corruption in their socieities", accomplished by being an even greater evil for them to band under: then he's treating the symptom, and not the cause. And the cause can be treated by introducing new ways of life with technology, that doesn't require existing countries' culture / infrastructure to rely on (like feudelistic societies "requiring" serfs, slaves, or plainly a social-status that practically exempts murder, etc).
That being said, the end result may indeed be that he'd be the ruler, or shadow-ruler, of all human society as a result, but I don't really see a problem with that. He's literally trying to manipulate all of the world's societies by being as hostile as possible towards everyone. The differnce is really not that big to make such a big deal out of it.
The reason why he is trying to do this by force, is because he is convinced that the core issue ,can't be addressed, not in a permanent manner.
You can't just remove the "corruption" that builds up in human societies, because it's not real corruption but rather part of the human nature that rises up due to fear.
They didn't execute Dwight and the heroine because something corrupted them, they executed them out of fear.
The King and nobles feared that the presence of those two will slowly threaten their own power and plans for expansion, and the common people out of fear that these two will take up the mantle of Demon lord and continue slaughtering humans.
And you have to agree, for the greatest part of human history, fear has been and still is one of the greatest drives for change,
humans created societies out of fear, fear of wild animals, fear of starvation, etc.
Dwight knows that he can't remove that fear from humans and as such can't stop "corruption" from rearing its ugly head again and again, so he opts to utilize said fear and use it to force them to live in peace amongst themselves, if for no other reason than that there is an evil Demon lord that can strike at any sign of weakness. And since he is immortal that can be a lasting peace.
I'll stop commenting on this subject with this one, and just say -- this is pretty much exactly the type of argument I was expecting to see. A tautological argument, that tries to explain how: "Humans aren't really evil, it's a weakness that makes them evil" -- so, they're evil because they're evil. Gotcha.
I've no idea where you sprout the axiom of: "You cannot change human fear, therefore, you cannot stop societal corruption and evil". You literaly have no basis for any of that, none.
If you argue that the basis of evil is fear, then address the reason that people, societally, will feel so fearful.
A person who is well fed, satisfied and has self-dignity, will not bear arms against another for no reason.
The reason Dwight and the hero died, is because they threatened the position of a ruler inadvenretantly. The issue is the infrastructure that pulls up violent, psychopathic people who thirst for power to the highest position, and are then unwilling to let go of it, and will murder and cheat, in order to maintain their position.
The solution is to absolve the systems that ENCOURAGE the worst of humanity to serve as leaders. It's quite literally as simple as that.
It's not human nature to be absolute scumbags, we are a herde species, our natural survival strategy is cooperation and commune. You can actually say our modern way of life, is actually the least natural our species has been....
And just like it's not in the nature of a dog or cat to murder a person on sight. An abused animal will be by far more violent than one that was taken care of.
You've done some major projection in that comment of yours.
TLDR: You cannot say that "humanity has certain tendencies, so you can't stop them eventually being evil" -- the issue will always be a system that pulls up the worst of humanity to the biggest roles of power, instead of communes that advocate for the benefit and wealth of everyone. Change the system, change the people.