Arin really is stupid, after the events of the blood banquet her own father asked if she knows anyone who’s connected to that incident and she had received a gift that made those zombie like begins target her from the first prince. That gift that her brother gave her, she knew that it was connected to the incident and all she had to do was tell on her brother and the king would’ve put him on a leash or worse execute him because a lot of nobles died so a light punishment wouldn’t have sufficed for the public.