My Theorie is that he gets to be the main force of the kingdom, the king thinks he commands him but in Reality the king is just used for his resources. As soon as he has taken over every position of humans in the kingdoms army he can simply kill the king while the story is that someone else toke the staff to take control of the golem army and then take the kingdom and place a puppet king as the new king while saying something like the staff is now blood bound to the new kings lineage so that nobody tries that again