If killing the minor Immortals that entered his Domain did not alert his enemies to the inherent dangers that were present in his Dimension, then the disappearance of a Demon King would do the trick, and they would not know what to expect. This should buy him a little time to push events forward.
He expected that there were extremely few methods to destroy a Demon King, and those of equal powers in the Great Abyss would have surely sensed the disappearance of such an esteemed figure. There were a limited number of such valiant powers in the Great Abyss, and the death of a single one would shake the Great Abyss.
He suspected that most would believe that the Demon King was trapped because he was not truly dead but sealed. It was the reason he did not go out of his way to kill the Demon King at the moment. Unlike any Immortals that he had slain, killing a Demon King would cause so much commotion that he feared it might attract the attention of a Primordial.
