Why King Yu sealed those demons instead of killing them is unknown, only he himself knows.
However, even though Jiang Chen didn't know, he could guess it on his own. He believed that it was a deeply strategic and far-sighted move for King Yu not to kill these demons. King Yu had foreseen that, after countless years, the Human Race would inevitably decline.
Thus, by leaving these demons alive and not killing them, he hoped that one day, his descendants could refine these demons to rebuild the glory of the Human Race during its decline.
These demons were the wealth left by King Yu for future generations, to bolster the foundation of the Human Race.
One who doesn't plan for the entirety cannot plan for the moment. King Yu, with deep and strategic foresight, had already been laying out plans for the future during the Ancient Times, leaving unimaginable wealth for later generations.
