"If you answer my questions honestly, I'll let you live for the simple fact that we're all humans."
Zhang Ruochen wasn't trying to trick the indigens of the Primitive World. He truly intended to let them live.
Why was it that only killing indigens above the Completion of the Earth Realm could be counted toward the military merits?
It was because Empress Chi Yao's wish wasn't to have the indigens massacred. She only wanted to control the large Primitive Worlds.
He himself was against genocide. He considered it the best situation if he could reduce the number of killings that he had to do.
If they could kill the top masters in the Primitive World, the First Central Empire could easily control this world and its indigens to build a new culture.
Thus, if the indigens were willing to go with the flow, unnecessary killings could be avoided.