The guy was pretty much already doing his best not just for his own subordinates sake, but for the entirety of the demon population, and they come at him with this. Might as well let them all have a go at themselves if they think he isn't doing good enough. Despite what they might think, there's probably quite a bit more to his job than "grr, kill all humans because I am demon."
As enjoyable and funny as this is. The characterization feels wack. Demon Lord lived for his people and wanted them to not be oppressed, now he's throwing it all away. He should know his brother will just create more division with humans and demons. This has so much potential (like every other reincarnation OP Cheat manga) but in the end it's just fast food reading with no captivating plot and fleshed out characters.
I think he means to deal with those that don't follow the king, but rather just want to make war with the humans no matter what.
Until they're gone, he won't be able to settle things down, so there's no point in being the king until then. And since he's the strongest, he can immediately retake the position whenever he pleases.