Being interested for a long time in the Zodiac killer and everything surrounding this case, I can admit I have read many articles and facts about it and I've come to a theory that could be wrong but it was fun to think about and I thought I should share with the rest of the tcc! Let me know what you think! :)
So. As I read in an article online, there were at least 8 suspects making the main suspect to be Arthur Leigh Allen. In the article, there was written why and how each suspect became one and why they were ruled out. For one of the suspects, there hadn't been a clue on why he was ruled out and his name was Ross Sullivan. Now, he either got ruled out but the writer forgot to type it in or he was never really ruled out but mainly ignored by the police or the people in general. Here are some thoughts that could somehow connect Ross to the Zodiac.
1. Zodiac was known for trying to draw attention and being a pathological liar. In many of his letters, he included things that were already written in the newspapers several days before he'd sent the letters and that is shown in the movie as well, commented by Paul Avery.
2. When the news said that the main suspect was Arthur Leigh Allen, the Zodiac made a phone call on December 18 to say "he wanted to kill because it was his birthday", that day was Arthur's birthday (shown on the internet and the movie as well) so the Zodiac made the police focus on the main suspect (who had drawn the attention by the police already but was not actually guilty for the murders)
3. In a phone call with a live TV show, the Zodiac said he suffered from headaches that would only stop when he'd kill.
4. The police found clues that pointed out that the Zodiac was in the military for some time (before the murders began).
5. The physical description was "tall, blond, strong, thick eye glasses and military style clothes and boots".
6. Ross Sullivan was born on July 28, most of the murders and letters started in July close to his birthday so he can draw the attention that he wanted like the police accused him of being an attention seeker.
Ross Sullivan was in the military indeed (no.4) and some of his friends described him as "weird and creepy at times". He would like to write dark and twisted poetry and wear military style clothes most of the time (including the boots that matched the footprints that the police had found on the murder scenes).
Arthur Leigh Allen got arrested in 1974 for molesting children and got out in 1977.
The Zodiac letters stopped in 1974.
Ross Sullivan died in 1977 due to heart attack at the age of 36.
Now, going back to the part (no.3) where the Zodiac said he suffered from headaches. Headaches and migraines are some times associated with heart disease, Ross (or maybe the Zodiac?) could have suffered from heart issues and that's what caused the headaches. The Zodiac mentioned they would only stop when he would kill, that could be because of the release of adrenaline or endorphins which help reduce pain in any part of the body.
Back to the part where the Zodiac tried to draw attention away from him and towards the main suspect (no.2) and the letters stopped; the media transmitted the news very fast so the Zodiac heard how the main suspect of the story got arrested and he had to stop the letters so that the police would actually believe that Arthur Leigh Allen was the Zodiac.
Do you believe a man who could use knowledge from different books to write cipher letters would be careless enough to get caught? Or do you think he could trap another man who happened to get dragged into the case under other circumstances?
And last but not least part! This is the sketch the police released publicly after the surviving victims stated how the killer looked according to what they managed to see
And this is a photo of Ross Sullivan (he must have been a senior then)
Again, this is just a theory! Don't @ me! Maybe I have forgotten things that could rule Ross out but there weren't enough sources about him and since he died in 1977, I guess we will never really know if he was the Zodiac or who could have been the Zodiac in general.