Because the demon lords are no more monsters than the human leaders we have been seeing. There's no such confrontation going on as you are proposing.
You might as well ask how on Earth can Germany and France both keep existing right next to each other, after they have fought so many bloody wars. Shouldn't one of them be permanently destroyed already?
He's not a just hero, though. He's the leader of a nation. A just hero would be a lousy leader, only marginally better than a tyrant. Of course in a paradise a just hero might be an excellent leader, but clearly the world of this series is anything but a paradise. So, Rimuru foremost must look after his own nation. How to treat other nations depends on how they act toward Jura. Friendly nations benefit Jura, so they are worth treating fairly, but hostile nations must be kept in check. If a verifiably hostile nation can be turned into a friendly one by extremely unscrupulous tampering with their internal politics, like Diablo is now doing under orders from Rimuru, then that's what Rimuru should do, if he can do it without risking the general situation becoming worse than it was. It's not an act befitting a just hero, obviously.