Si yan zai (1985)

Lee Kam Lun (Alan Tam) is an intelligent but psychologically unstable young man who is very filial and loves his mum(Pik Wan Tang) dearly. Trouble brews when he accidentally killed a junkie one night. His subsequent actions arouse the suspicion of another intelligent Inspector (Bill Tung), who kick start a game of cat and mouse by befriending Lee's mum as he tries to expose Lee as the killer. Director Ronny Yu (Fearless, The Bride With White Hair) gives us an effective and tight psychological thriller with some dark humor and plot twists in one of his earlier directorial effort. Cantopop legend Alan Tam and the late Bill Tung both give outstanding performances playing the two key adversaries.