Why did Shang Hongfa kill Xiao Gu's mother?

Because Xiao Gu's mother knew the secret of "him" according to the query of iQiyi app, in Dawn, "Shang Hongfa killed Xiao Gu's mother because Xiao Gu's father killed his father, and Xiao Gu's mother was also Xiao Gu's wife, so Shang Hongfa thought that killing Xiao Gu's mother was the best way to get back at Xiao Gu's father". The play tells the story that Xu Charlie, a barrister, resigned because he couldn't bear the shady anger of all kinds of money and power transactions, and went to the poorest slum in Jiangzhou, a major town in the south of the Yangtze River to open a law firm for the poor who couldn't afford a lawyer. In the process of confronting evil forces, Ling Xiaogu, a female detective with a sense of justice, met Charlie Xu, and they gradually fell in love.