Hostile Takeover of a Corrupt Kingdom

For generations, the human kingdom has prepared for an invasion by the Demon Lord. Its prophecies predict black armies, burning villages, corrupted forests, and a climactic battle between the Demon Lord and a divinely chosen hero. Instead, the demons summon an unwilling human from earth to beg them to serve as the demon king and defeat the corrupt, violent, and powerful human kingdom that keeps killing their people in ridiculous ways. He is warned that directly attacking the kingdom leads only to despair and doom. The kingdom can destroy the demons at any time, but keeps them around as a scapegoat to blame for all the hardships their people suffer in order to distract from the kingdoms corruption. The demons heard earthlings were very good at economic manipulation, creating false narratives, and dividing populations against their leaders. If you attack the human kingdom openly, you will lose. You must find a way to manipulate the humans to rise up against their leaders, both political and religious. Only then can you engage in open warfare against the kingdom and win.