It was just some weirdo worm in a rotting prison. Zul says something like "Blood is our path now, the path of G'huun". Was G'huun whispering to him like other Old Gods? But he was just some experiment by the Titans, not even a "real" Old God. Why did people want to follow G'huun? Zul wasn't an idiot. He was pretty smart. At least up to some point. He was a prophet. He would plan things through very thoroughly. Like he said, planning even for the future outcomes he did not foresee. We saw that when he espaced from Stormwind, setting fire to the city as they escaped. in anticipation of the confrontation with Jaina, so they could get away. We saw that in the first few quests in Zuldazar. And then he followed this worm? What was he thinking?