First, Pound's son Ou XIV killed Zheng Shusen's daughter-in-law Qin Shanbao in name. Qin Shanbao's death made Zheng Shusen decide to kill Pound and avenge Qin Shanbao. This directly led to the opposition between Zheng Shusen and Pound.
Secondly, Pound is a traitor. Zheng Shusen thinks that he killed Pound for the honor and interests of the country and the nation. At this level, Zheng Shusen must also kill Pound to safeguard the interests of the country and the nation.
To sum up, Zheng Shusen wanted to kill Pound for both personal reasons and national reasons.