There are a lot of issues here.
Sunghoon decided on his own what was best for Rangii, he doesnt think she can be the King of the Phantoms and never asked her opinion, a lot of this is because Rangii looks as a child when she is actually very much older but acts immature because she was most of her existence isolated from contact with other ... people.
Rangii took offense because she been King of the Phantoms for a very long time and genunine wants to help Phantons, yes ... Smokey used that in a attempt to drive a wedge but this is were Rangii immanturity is causing problems, as much Sunghoon shut Rangii out of how to solve the problem, Rangii is doing the same to him and both are taking the other as granted as if Rangii would just stay in her room or that Sunghoon is just going to wait at home until she is done.
Who is at fault? both for doing the same thing to the other, we could say Sunghoon started by trying to solved things on his own but Rangii also acted as she was the only one to make the decisions cutting off Sunghoon from something that affected both.