A Whetstone With An Independent Consciousness

The reason Fan Xian didn't kill Hu Jinlin was simple. The Third Workshop had always been managed by the palace treasury with the Third Bureau of the Overwatch Council. They were specifically responsible for the research of advanced military technology. Furthermore, the Third Bureau of the Overwatch Council was originally Fan Xian's brother- and sister-disciples, and he was particularly knowledgeable about the situation of the Third Workshop. Hu Jinlin was focused solely on studying the blueprints left by the mistress of the Ye family; his personality was silent and dull. Although he had gained a substantial amount of silver through corruption, he had never done things that Fan Xian could not forgive such as forcefully take land or take advantage of women.

Compared to the First and Second masters, there was indeed reason not to kill him. The most important reason was that Fan Xian didn't want to kill.