Killer does not need to have reasons to kill.

Human is full of emotions. In many occasions human is driven by emotion outbreaks. For (young and proud) girls, jealousy might be the worst poison. Zhu Ling does not have to do anything. If the killer likes a boy and happenedly the boy likes Zhu Ling, this forms a very good reason.

This is such a tragic story. We do not know the fact and we will never know. No suspect. No evidence. No nothing. We could accuse Sun Wei but even this accusation might be fatuous. We just do not know.

Blame the law enforcement agency. They did not do their job.