It still wasn't, at least before Marianee decided to make a move against his orders. For William, the act of betrayal that the concubines would commit was something he could not forgive.
Therefore, William would not hesitate to enslave them and then subject them to the influence of the Nether Stone. With William successfully enslaving the concubines who tried to betray him, William's problems in the future would be resolved in only one way.
The reason why William didn't enslave Yelna was that he didn't want to waste his time enslaving a woman who was no longer a threat. But that wasn't the main reason why William didn't want to enslave Yelna. Even if Yelna had not completely submitted to William's orders, she had previously achieved half of the enslavement effect that the Nether Stone had on her.
