Who Killed Cop Robin?

Burke's mentor is slain in a dark alley. His killer may be a suspect from one of his former cases who got off, but who was actually guilty. But why would Officer Robin confront the killer without backup? Can Burke decide which (if any) suspect previously accused of a crime (possession of narcotics, assault with a deadly weapon, murder, blackmail and smuggling) is Cop Robin's killer? Burke discovers that the victim was already dying and was trying to prevent another crime from being committed by forcing the criminal to get caught for his murder.