WMG / Zodiac

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.
Zodiac was a drug addict.
I'm thinking like the "galaxy of multi-coloured pills" from Fear and Loathing in Las Vegas. Acid would explain his loony letters... I don't think he believed any of that brew about "the people I kill will be by slaves in the afterlife, etc..." I think he was just off his face. Probably before killing people he needed maybe some kind of upper to give him the nerve to go through with it. Obviously, this doesn't explain his motives, or his use of cyphers, I just thought it would explain all the kooky nonsense he goes on about. Or, maybe he was just really, really, crazy.
All the crimes are indeed connected because Zodiac was deliberately trying as many Modus Operandi as possible.
Or, in other words, his was a macabre attempt to be a Post Modern nightmare. He started out with a random murder of one person in 1966 before ambushing a couple (the two crimes not seen in the movie), then acting out being a stalker (hence why Darlene seems to know him), then deciding to copy Jack The Ripper by sending cyphers and 'trophies'. The creepy outfit is yet another random affectation - the obsessed fan of a work of fiction. The taxi murder was a self-imposed challenge to see how brazenly he could act and escape. He tried to do a kidnapping but the failure meant he gave up and disappeared back into obscurity. The plan even included trying to frame up someone else for one of the crimes; perhaps all the circumstantial links to Arthur Allen were because the Zodiac met Allen in passing, followed him around, noted his 'Zodiac' watch, his "most dangerous game" talk, and even timed his murder to match Allen's camping trip.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/Zodiac