Ben Hong did not mean to spare Chen Rui, but from the standpoint of profit, she currently needed Chen Rui's power to enter the Light Holy Mountain once more and comprehend the Source of Creation. After all, that was the most important purpose behind her years of lurking and her eventual arrival in the Surface World.
Chen Rui's desire to kill Ben Hong ran even deeper, because she had already threatened the safety of the family and friends he treasured most in the Demon Realm. His cooperation with Ben Hong was not for profit, but forced by circumstances. Neither Light's Blazing Thunder nor the Holy Grail could truly control Ben Hong; they merely added weight to the bargaining chips in his hand, allowing him to secure the best possible outcome given the current disparity in their strength.
As for the Tower of Glory, it was an unexpected gain, and likewise a stepping stone for cooperation between the two sides.
Ben Hong proposed continuing to reside within the six-star mark on Chen Rui's wrist, but Chen Rui refused on the grounds that it was inconvenient for his cultivation and daily life at present. He only said that she could reside there again once the time was right to head for the Light Holy Mountain. Chen Rui also tentatively steered the conversation toward an equal contract, only for Ben Hong to calmly shift the subject away. Having personally witnessed Chen Rui dissolve a pseudo-god-level contract and the strange power of that "Divine Kingdom," Ben Hong had at least lost her former lofty, condescending attitude toward him in terms of both ability and scheming. She now regarded Chen Rui as an opponent who stood almost on equal footing with her.
With the events at the Holy Mountain as a lesson from the past,