The Amidonian King is a moron too. Dude takes all the country’s budget and pours it ito military spending, intentionally keeping the populace hungry so they have something (MC’s country) to resent. He even planned to have dissenters executed, so all the same government officials fled.
Basically, the MC always comes out on top because he keeps various philosophies honored and maintained, whether they be Machiavelli or the Art of War, even taking pages from lessons learned from the failures of Communism and the Cold War...
To sum things up, everyone will always be conveniently stupid so the author can have Souma quote Socrates or someone and come out on top.
As for the Lion General dude, he did it under the orders of the previous king, who had married into the royal family. The queen actually has slight time control magic, and has been Re:Zero’ing for some future where Amidonia doesn’t spank her country into destruction.
Her last run almost went well, but because Souma didn’t fully punish he corrupt aside from extorting them, they lived long enough to literally set the capital on fire, with Souma presumably perishing along with her daughter. So the plan literally settles what Souma failed to see through.