According to Conan's perennial theorem, when all the evidence points to one person, then this person is definitely not the murderer. Therefore, Liu should be the scapegoat set up by the screenwriter to divert our attention. Although there is not enough evidence to prove who is the murderer, I guess the murderer must be a seemingly harmless character, so as to give the audience a sense of excitement of "Wow, it's him". Moreover, the murder case by the river revealed that the infantry was the murderer, and the infantry and Han Tianfeng were mentoring. Later, Han Tianfeng participated in the abduction of children, and he paid special attention to this matter. Combined with his tragic childhood experience, his psychology must be distorted and his values subverted, so it is understandable to be a scavenger. So, I think this Han Tianfeng is a street sweeper.

Why is he a clue to the whole play? Clues are soul figures. From beginning to end, he has the task of promoting the plot and stringing all the plots together. The play revolves around the street sweeper from beginning to end, so he is a clue figure.