Consider the case carefully. After reading it for a long time, I don't understand it and need an answer.

Girls actually like boatmen very much. The boatman knew Li Si's proposal, so he found Zheng Wu and used Li Mu's superstition to cut off the doomed love. But the woman was greedy for money and didn't agree, so Zheng Wu had to let them sleep together. Because he didn't let anyone near him, only he had the key, so the boatman took the key from him and killed Li Si. The man who came out was not Li Si, but the boatman. He didn't know who it was because he covered his face. Boatman He was afraid that Zheng Wu would be exposed, so he killed Zheng Wu, so the body in the lower reaches of the river should belong to Zheng Wu. Zhang San is a very clever man. He didn't know the whole story until he heard about Li Si. Because he has no evidence, he can't directly tell who the murderer is, so he can only secretly help his brother, pretend to be a ghost and tell him to go back to find the box.