Putting it simply, what Heavenly Fate had been doing was researching the human body and creating superhumans.
Instead of calling what the nightcaw birds had been transmitting poison, you might as well call it drugs to stimulate changes to the human body.
It was survival of the fittest in natural selection.
In the end, each subsequent transmission of the drug would breed the strongest person. Those who adapted would evolve more rapidly, while those who didn't could probably only turn into zombies as a foundation.
From the looks of it, the experiments Fan Qiuhe conducted on a small scale had similar purpose to Heavenly Fate's deeds.
They were both researching the human body. However, Fan Qiuhe was probably only doing research for his own benefit. He wanted to obtain information on how to become strong from other people's bodies. This included how to augment one's inner world and spiritual conscious, etc.
