People have speculated on this for years. I never really believed that Zodiac was two or more people. I was always looking to pin it on one guy. I no longer believe in the lone killer hypothesis. For one thing, Graysmith mentioned that the killer must be immensely powerful because he has tossed bodies over fences and across wide ravines. Two people, though, could do it with relative ease.
I always rejected the Lompoc killings as Zodiac because I was certain Cheri Jo Bates was his first victim the way he couldn't stop crowing about it to anyone who'd listen. But there are earlier killings that are uncomfortably close to Zodiac's MO and signatures. Like you, I posit an older killer recruiting a younger one. I am positing a possible cult. A devil cult or something like that. Clearly, there is some occultism involved. Pam, sister of victim Darlene Ferrin, stated to Graysmith concerning Zodiac, “Darlene mentioned the witchcraft angle to a certain degree, knowing this guy that was sitting at the counter up at Terry’s, just the way he talked, the way he was into that skull with the dripping candle. Darlene said that he was the one who kept it lit and talked about these weird things and witchcraft."
The killer stated in his letters that he was killing people in order to amass slaves for his afterlife. Certainly, not a belief of any organized religion. There are tribes in the Indochina/Southeast Asia region where such beliefs can be found. It is clearly a tribal belief initially. A warrior of his tribe wants to die fighting for and defending his tribe and the belief may arise that the more men a warrior kills, the more slaves he is collecting for his glorious afterlife. This belief can be found, apparently, in the Philippines. This is interesting because there is the prevalent story of the Vallejo bartender who kept trying to date Darlene but she always refused. He was reportedly quite upset about this and would come around her place all the time trying to get her to go out with him. Turns out his name is George W. Waters and he is Filipino and was born in that country. He was about 5' 8" and somewhat paunchy in build. So, that made him a person of interest but Waters was dark-skinned with jet-black hair (many mistook him for Mexican) and so does not fit any descriptions of Zodiac.
Sorry for going off on a tangent. But we see anyway that Zodiac is motivated by occultic beliefs. He killed victims on or near auspicious days of the year and holidays. It would appear that he recruited a new member and perhaps this new member had to kill somebody in order to be accepted. This person then stalked and killed Cheri Jo Bates and bragged about it because it was his first kill and it got him into the club, so to speak. Earlier killings as Lompoc were done by the older one. He also appears to have written most of the letters although the younger member might have been allowed to write one or two besides the Bates letters. That would explain why the Bates murder seemed so important to a killer who had apparently already killed at least a few times and the urge to brag should now be muted.
This would mean that the Lake Berryessa attacker was older one whereas the others (Faraday-Jensen, Ferrin-Mageau, Shepard-Hartnell, Stine and Johns) were done by the newer member. The Berryessa attacker weighed about 220 lbs but the other attacker isn't anywhere near that heavy. The Berryessa man had dark-brown, greasy hair where Mageau said his attacker had "champagne-colored" hair, that is, it was kind a light brown with a hint of orange combed high and back into a pompadour. Since the Berryessa attacker wrote on the door of Hartnell's car and it matches the handwriting in the letters, then he is the senior member of the cult. He carried out one attack himself for whatever reasons and ordered his follower to do the rest. Then he would write the letters.
Zodiac did brag that he had the ability to look entirely different when he went of his hunts. I'm sure he used disguises but he was probably coyly admitting that there is more than one killer.
