The premise for the murders is that the murderer is hypnotized to kill upon hearing a musical piece being played on the piano. It has been proven time and again that hypnotism cannot be used to make anybody do anything against their will or act while in a trance and not remember the events afterwards.
Even in the rare instances where a subject will accept a posthypnotic suggestion, when the trigger is used they, at most, get an easily controlled impulse to complete the implanted action. At which point they become fully aware of what they had been told to do, and that they had been told to do it.