Zodiac only has the urge to kill when Hurdy Gurdy Man comes over the radio as in the first scene.
He's got Chronic Villainy
and is the Pavlov's Dog of serial killers.
- Except it wasn't playing during the taxi shootout, or the attack on the third couple.
Robert Graysmith is the Zodiac Killer
Robert Graysmith has been keeping the Zodiac Legend alive. The book, the movie, everything. Zodiac killed for recognition. It's why he sent letters to the police. He craved the attention. He even asked "who will play me in the movie." An eventual movie was made off of Graysmith's book. Graysmith injected himself into the investigation, which is precisely something that Zodiac would do. He had access to all the files. Zodiac stopped killing because he didn't need to any more. He could become known as Robert Graysmith, which would feed his narcissism better than being known as Zodiac. Because then people on the street would recognize him, whereas nobody would recognize him as the Zodiac killer. Zodiac was a fan of ciphers. One of which is still unsolved. The reason it's unsolved is because it's Gibberish. This was to throw police off, and add to the urban legend: an unsolvable cipher from an uncatchable serial killer.