Priest

because he want the black hair to be emperass and rule the nation by making both the king and queen his dolls

Plot and drama reasons only. Because someone that smart would have investigated just two seconds and would have found out Robellia wants put and would have just helped her with that. Way easier than having her killed and having the emperor investigate and less drama inducing

doesn't he actually have a motive to kill robellia? he wants to kill her, not strip her of her position as an empress. he could've just sent her back but it's either that he's lying or he doesn't want to.