Obviously, Xin Feng's plan wasn't that simple. Of course he would benefit from it…
What nobody knew, was that Xin Feng's will was indeed his will, however, it was not just the will of him wanting to create the pill. It was an actual part of his consciousness, which would allow Xin Feng to leech off of other's soul, and even possess them. In case anyone were to betray him, he could always possess their soul, and if they didn't betray him, he would simply absorb their energies steadily. With an entire population that would have a mark of his on their soul, Xin Feng would be able to get his source of power from an entire country. Xin Feng was that despicable, and that far ahead with his plans.