The Major Weakness Of Gods.

Zernon has been made practically immortal by the will of fate. He is the lovechild of Order after all. Nothing would have worked to kill him deliberately had Zerkon not seen how to make Zernon fall. It had to be a betrayal that occurs when Zernon is at his absolute lowest. So Zerkon had to engineer a betrayal or several betrayals. His schemes have led them to this point. Now, Zerkon has to make Stelios finish the job that he started and kill Zernon.

He could have taken a more active part in the scheming but Zerkon is an observer, he can only look not interfere. It is the side effect of his abilities, he can't kill Zernon even if his Celestial Authority is strong enough to do it. Someone else has to fire the weapon and that person has to do it without being influenced by Zerkon.