I have two theories
First theory , MC is going to find his decapitated family, likely with heads on spikes like they usually do in revenge based plots like this, and will wipe out the kingdom or at the very least the king . Due to him having put out info that he was going to summon the hero, the other kingdom will think that he is the demon lord for sure and is trying to stop the hero summoning/training.
Second theory, hero turns out to be evil. Whether he starts out that way or is influenced by the king IDK. Real demon lord is nice and trying to do same thing as MC and live a nice slow life avoiding their role as demon lord.