Zhu Honggong, Old Eighth, would be valuable to Lu Zhou. He would be able to gain merit points from Old Eighth before he disciplined him as well. Apart from that, Old Eighth rarely committed crimes on his own accord. Usually, it was Si Wuya, Old Seventh, who instigated Old Eighth into acting. It was rumored that Old Seventh's Darknet was located at the Crouching Dragon, and yet, nobody had ever found it. All they knew was the Darknet was everywhere and their network of information was very wide. It would be difficult to catch Old Seventh.
Moreover, Lu Zhou had more pressing needs. He needed to improve his strength first and foremost. Rumor was rife that he was going to meet his end in ten years. Everyone was just waiting for him to die.