Without going too deeply into spoiler territory, while there's still a lot of questions left to answer, the latest MSQs do quite a bit to explore the motivations of the Ascians. Suffice it to say, destroying the world is NOT on their agenda. In fact, doing so is considered to be an embarrassing faux pass among their kind. So, it doesn't make a lot of sense that individuals who want to destroy the world be recruited by Zodiark.