The killer is never explicitly revealed, but it is strongly implied that Tom Cutler is responsible for the murder. Throughout the movie, it is evident that he has a devious and murderous mind, and is able to conceal his true intentions very well. Additionally, Cutler exhibits several key characteristics that make him a likely suspect: he is highly experienced in law enforcement, knows how to cover his tracks, and has a history of violence. In the end, it comes down to circumstantial evidence - and ultimately, the viewer's opinion - to determine who killed the victim in cleaner.