I don't really remember any flashback on how he is the king? He just was from chap1
They never really say just they died by unknown reasons and his dad. So by default he became king I guess. But I think it was just assumed he planned to kill everyone( he's crazy enough to go to any length to obtain MC. By becoming king he finally had the power to summon the MC without refusal. Plus, the absolute authority to confine and do whatever he wanted to him without question. I'm quessing the older brother was cunning and caught wind of this beforehand. So he faked his own death to escape and after regaining some strength/ power returned now to reclaim what is his.
wait, I'm lost, how did he become king again? (if someone knows, do tell me, I don't really want to re-read it)