Fang Zheng disliked the Immortal Sword Sect for the same reason he could not go on a killing spree in the Pitch-Black World.
The reason was simple: he could not do it.
Just as Fang Zheng had told Lin Yueru and Zhao Ling'er, even if he killed every monster in this world now, after a few decades or centuries, those meant to gain sentience would still gain sentience, those meant to become monsters would still become monsters, and those who died unjustly and were destined to turn into evil ghosts would still turn into evil ghosts.
The Pitch-Black World was the same. The Cursed Children were indeed pitiful, but could Fang Zheng really unleash the Swarm and slaughter every adult in the Pitch-Black World, leaving only the Cursed Children alive?
What about those who had just been born? What about those who had yet to be born?
More importantly, the Cursed Children would not accept such a solution either.
To solve this problem completely, a comprehensive plan was needed. Just as Fang Zheng had done in the Pitch-Black World, he had not made the Zerg slaughter all the adults there. Instead, he had used the Swarm to control that world's human leaders, enact laws favorable to the Cursed Children, and gradually create an environment where they were no longer discriminated against.
But the Immortal Sword Sect's approach was to push all monsters onto the opposite side of humanity. It was like Cerberus and the Flame Haze Warriors: if Cerberus had the ability to wipe out every civilization in the entire Milky Way Galaxy except humanity, just like the Reapers, then the Stellar Alliance would definitely collude with Cerberus.
The Flame Haze Warriors were the same. If they had the ability to wipe out all the Denizens of the Crimson Realm