Many viewers mistake Lau’s arc for simple guilt. It’s much darker. Lau is suffering from (a form of split personality) brought on by traumatic brain injury and extreme psychological stress.
The emotional core of the film is Lau's descent into madness. Haunted by the death of his rival, Chan, Lau begins to suffer from : He hallucinates that he is Chan Wing-yan. Infernal Affairs III
The Final Descent: Navigating the Maze of Infernal Affairs III Infernal Affairs III Many viewers mistake Lau’s arc for simple guilt