So basically: "The demon-lord method is flawed, I experienced it first hand. There is no such thing as peace."
And then: "The only logical conclusion is that I must be the demon lord!!!"
What.
No need for that pseudo-logic wank. Just say: "I am insane from anger and the literal death-curses of hundreds of people are swimming in my head, and I want to kill things". There. Done.
If he actually wanted to do anything helpful, he'd
1. Take total control over the monopoly of force of each, or most nations. He's the only one who's allowed to enforce the right to harm others, and relegate it to others.
2. Wipe out the concept of a ruling-class, and spread equal treatment for all citizens. Extra points if a meritocracy can be achieved.
3. And probably most important of all: Use his undead to create a workforce that does not require payment nor food. If the undead curse the land and thus cannot "work", I'm sure a fucking Sage can think of something productive here, even as an undead. Magic is magic, and surely he can use his new founded nation to find the human resources that'd do it for him, ffs.
As such, the worth of human life is reduced to shit, which means they're not worth the hassle of enslaving or lording over.