Street madman Act III Who is the murderer?

Defa finally defeated all the boys in the village. Gou Fugui encouraged Wang Defa to continue to challenge Wu Tenglan, the daughter of the village chief, but was defeated by Wu Tenglan. In order to get back at Wu Tenglan, Gou Fugui spread rumors and let the boys in the village isolate Wu Tenglan. Finally, at the age of marriage, Wu Tenglan still failed to get married, so her father Wu De helped her to open a steamed stuffed bun shop in the village for the time being. Wu Tenglan put his hatred for men into making steamed buns. The steamed buns were delicious and soon spread all over the village, so he was called Baozi Niang. 1997, Zhou Tieniu, an old charlatan, came to Hongxing Village with his apprentice Li Gou, who was collected in Yunnan, and pretended to be a magical doctor "Saibaiwei" to cheat. At the same time, Wang Defa is also jealous of the popularity of Wu Tenglan steamed stuffed bun shop. At Gou's instigation, he pretended food poisoning with shampoo in an attempt to discredit his reputation, but he was caught and beaten up. Gou recommended Wang Defa to see a doctor named "Sai Budweiser". In order to attract attention and promote marketing, Zhou Tieniu injected Wang Defa with expired Huoxiang Zhengqi water with a giant syringe, which led to Wang Defa's illness and death.