The Demon King seems like a really good King, pretty much exactly what you would want in a King, he is capable of making rational decisions, doesn't get held up by meaningless pride and arrogance, truly loves his people and thinks about what is best for them when deciding on a course of action. (similar things seem to go for the Princess)
But naturally even with such a good ruler there has to be some shit for brains in a position that has influence. I mean what kind of train of thought is that even :
"What ? He doesn't plan on risking his entire military power for a temporary victory that could be turned around in an instance ? Instead he makes the rational decision that will protect and benefit all of his people, while also keeping his entire military power unscathed and ready to take action, by first trying to take the person that could singlehandedly turn their victory into a defeat out of the equation (and if possible even get him on his side) ?"
"What a weakling, we should overthrow him and then run our entire army into the meat grinder, leading countless powerful demons to a meaningless death and leaving our people vulnerable, because that's how strong people do it!"