Zell discovered that after he evolved his race, the quality of his blood and fluids increased greatly, and any humanoid or beast that took in his fluids would have a high chance of surrendering to him. Thus, he planned to use these to slowly create a powerful guild of non-humans and NPCs.
'I wonder if my thinking is wrong; let's consider helping people like that boy and his sister. They should probably be quite strong by now. Let's have Cynthia look for them...'
His reason for choosing NPCs over players was simple; the players had ambitions and hopes, and they were yet to experience the true horror of the world after merging.
The NPCs have all already lost their homes; knowing the terror of demons, would they truly act as arrogant as humans? Plus, his way was much gentler than the so-called gods.