The reason why Fang Zheng dislikes the Sect of the Immortal Sword is actually the same as why he can't unleash a slaughter in the Black World.
The reason is simple: it's impossible.
Just as Fang Zheng told Lin Yueru and Zhao Ling'er, even if he kills all the Yokai in this world now, after decades or centuries, there will still be new Yokai and Evil Spirits from wrongful deaths will emerge.
The same goes for the Black World. The Cursed Children are indeed pitiful, but can Fang Zheng let the Swarm kill all the adults in the Black World, leaving only the Cursed Children?
What about those just born? And those yet to be born?
More importantly, the Cursed Children wouldn't accept such a method.
