If one were to ask the subjects of the Demon Clan to speak truthfully and to give their thoughts on their Demon Emperor, it's likely that each subject would say, "Our Demon Emperor is a self-important Demon."
How could it come to pass that he has not a single son, if not for the Demon Emperor's self-importance, believing that those around him dare not defy him?
He didn't believe that the Crown Prince had the courage to usurp him, which enabled all of this to happen.
Even so, the Demon Emperor never considered his excessive pride to be a flaw. After all, even though the Crown Prince's plan to leave him sonless was successful, wasn't the Crown Prince eventually killed by him?
Because of this, the Demon Emperor believes that his imperial authority is unshakably secure; he feels that no one dares to challenge his authority again.
An emperor who can personally kill his own biological son—who would dare to play any tricks?