Even with this chapter I still can't really see the "betrayal" angle, like his brother calling him back after he's been crowned is logical, and sending a Holy Sword wielder to fight against other Holy Sword wielders is also logical, like what is MC even planning to do if doesn't want to fight the empire? Stay in the Frontier town and hope the empire doesn't invade them? By that point it'd be too late to have a fighting chance against the Empire.
I honestly feel like the "betrayal" angle is cobbled to justify making the witch MC has as a companion be into kinky stuff with kinky powers...