It's implied he's been serving G'huun since around cataclysm, which, was also when the Nazmir seal was broken. G'huun promised a new Zandalari empire, and frankly, could've delivered. If G'huun got out, that infection might've been enough to kill even the Old Gods (via killing the planet and starving them).
- - - Updated - - -
Also what? Most got out? There were 24,000 people there, and we save like 50.