context: Ling (Aaron Kwok) is a married self-assured Police Chief Inspector with an adopted 5-year-old son who is investigating a cold-blooded serial murder case. However, due to the homicide attempt of a fellow inspector named Tai in which Ling emerged unscathed, colleagues are fast becoming aware of Ling's potential involvement. Ling suffered a loss of memory since that incident and cannot recall the events leading to Tai's attempted murder.
  As Ling sifts through the clues, he finds that all the evidence is pointing toward himself as the murderer. However, Ling's belief and unwavering sense of innocence fuels him to maintain his suspicion and keeps digging. Nevertheless, his colleagues do not believe him and he accidentally kills his partner in an argument.
  Ling takes a temporary leave of absence and enforces a lockdown mode to safeguard his family. During the lockdown the central plot twist of the movie is unveiled, as Ling's adopted son confesses his real identity to him. He is, in fact, Ling's half-brother, born by Ling's father's mistress who was abandoned by the father. Driven by a life of pain, humiliation and suffering for him and his mother he vowed revenge and painstakingly made his way back into Ling's life as his adopted son. He was able to accomplish this because of a medical condition where his physical stature remains that of a 5 year old. Having shared a bed with Ling's wife and paid close attention to Ling's friends he is able to slowly fulfill his revenge and give out orders for the murders which was carried out by his accomplice, a mentally handicapped man he befriended years ago at a disability center.
  Ling's wife is murdered in their home, driving Ling insane. Ling is able to gather himself to kill the accomplice and is moments away from carrying out justice on his adopted son until the police arrive and shoot him. The end of the movie shows Ling's boss discovering that the adopted son is in the picture containing Ling's childhood friends and begins to suspect something whilst Ling is in a mental hospital engraving the words "Revenge upon release" on his left arm. Meanwhile, his adopted son is depicted on the beach as a maturely dressed child contemplating the future.[1]
question: Who shot Ling?
answer: police