And you see, he knows once he's gotten Ryuu in that state Ryuu is helpless. Only his lovers calmung voice could help Ryuu, bring his mind back to clarity. But if buddy keeps Kashi separated from Ryuu then all he has to do is keep him in a in closed room with oh, say a few dangerous objects or jagged walls and watch as Ryuu destroys himself. Buddy also knows Ryuu won't care at that point and try to kill him. But the demon doesn't care if buddy dies either. So hence Buddy doesn't care. He just wants to watch Ryuu suffer ghost or not.