Damn, I almost got swept away by the entire comments that belittle this manga! Even 1 review on MAL not recommended this manga. All of them not liking the idea of "no killing" rule, while the main role of this MC is a fucking Phantom Thief who steal everything for his own gain. They quick to judge just based on 3 chapters and drop it, a shallow mindset indeed (or edgy boy mindset that want to kill everything, cliché edgy mindset etc etc)
Glad I continue reading and satisfied by MC's decision, the revenge is satisfying (for the king and the 3 idiot heroes). He was part of a band of thieves when he was human, and he stick to his core as a thief even after becoming Demon Lord (he stole from corrupt people and distributed it to the people he cared about). He didn't kill indiscriminately, because what can be stolen from a dead person?
He steal everything -> then spare the target, let them live in humiliation (they who once had everything, was powerful and arrogant, now loses everything and must live in misery for the rest of their life) -> even if they regain their power and turn against him again, he just steal everything from them again, the cycle repeat. It's incredibly satisfying, and I like this manga.