Lord Zealot was a man of great ego. So he revered being worshipped by the people for getting rid of the witch leader and tried as much as possible to hide how paranoid he was feeling, especially with the strange deaths of a few elders. However, with the birth of Harold, everything became worse for him, as he could feel his influence dying.
He had also taken his anger out on Arya. Even though she was the queen, he had assaulted her a couple of times, including hitting her for not bearing an Alpha. Then imagine what he did to her when she gave birth to not only a girl but an Omega.
Unfortunately for him, it was difficult to control things when he wasn't very sane. He needed to live a long life to be able to shape the kingdom back into what it was supposed to be.