idk man, I think the pathetic death fits his character well.
From the beginning we know that the Black Demon Lord prefers to do things in an underhanded way, so him trying to weasel out of the lynching through a War declaration is in character. MC even denied the War but rules are rules, as much as he doesn't want to do it, but when some higher being is in control he can't exactly refuse to participate.
In the end the way the Black Demon Lord died is pathetic. Lost everything due to his underhanded methods and with no lifelines left. There's a good chance someone killed him, but it doesn't take away from the fact that it is a pathetic death, a perfect representation of what he is.
Would it have been more satisfying to lynch him? Maybe. But dying off screen after losing everything fits him as well.