Hahahah! This is so bad that it's actually good.
Why would the prince reveal himself as a traitour immediately after suceeding the throne? He could just have pretended to have come to an agreement with the demon tribe after that. That would have helped to avoid revolts and strengthen his standing. Specially if he is going to impose measures such as MONTHLY DWARF SACRIFICIES! Well, we all know that the author made it like this so the MC had someone to punch at his arrival instead of continuing with the intrigue and the deceivement.
And talking about the MC, why in the heavens would he come in bursting through a wall? That seems like a great way to accidentaly crush people under the rubble. He didn't try to communicate first in anyway, though he carried the first prince with him? How come no one warned about an incoming flying vehicle or tried to swat it out of the air? Was it moving so fast? Was it invisible? Does the MC emits an aura that dumbs down everyone around him? We will never know.
This manga has never been a bastion of consistent writting or logical thinking, but damn, it keeps besting itself.