Who was the murderer among the seventh reader of the book?

There was no clear indication of who the murderer was among the seventh reader of the book. The plot of the novel gradually revealed the truth of a serial murder case through the perspectives and conversations of multiple characters. In the story, everyone had their own guesses and doubts, but they were unable to determine who the real murderer was. It was not until the end of the story that the truth revealed the identity of the murderer. Therefore, the readers could deduce who the murderer was based on their own understanding of the plot.