Two murder cases with vague motives and clean crime scenes were linked by a phone card. Yuyang City Criminal Police Li Shang launched a series of investigations into the two cases, but made no progress. When all the clues were lost, Li Shang discovered that this was a serial murder case involving the 2007 mass murder case. He investigated all the suspects from the perspective of revenge killing, but failed. Who is the murderer?
Purpose: Caring for the mental health of young people