I recently read "Bind Torture, Kill: the Inside Story of the Serial Killer Next Door." There were lots of parallels to the Zodiac case. Here’s one that jumped out:

Some of the cops, including Landwehr, were surprised when they finally got a good look at BTK. They had assumed he would be smarter. Back in the 1970s, some cops had theorized that BTK was a criminal mastermind who disguised his intellect with crippled sentence syntax and intentional typographical errors. The timing of the "Shirley Locks" poem appeared to be connected to a contest in Games magazine. Local Mensa members fussed over that possible connection for years, convinced that BTK was brilliant. But it had just been a coincidence of timing.

After talking to Rader for hours at a time, the detectives concluded BTK was more lucky and stupid than smart.

Exactly the same thing happened with Zodiac. Some people are convinced Z is a master criminal, a genius. Could he be a genius? Sure, it’s possible. I don’t think he was a dummy. He showed some signs of intelligence. But I don’t think anything he did required him to be a genius. You don’t have to Lex Luthor to shoot people in the dark and send misspelled letters to the newspaper.

For example, none of the things people take as signs of Z’s technical abilities are very hard to do. It doesn’t require genius to tape a flashlight to a gun barrel, to copy a bomb diagram from a book, or to do a dead-simple, textbook substitution cipher like the Z408. If it’s proven the Z340 is a legitimate cipher that defied the world’s greatest cryptographers for 50 years, I’ll change my opinion so fast it will make Zodiac’s hood spin, but for all we know the Z340 is just gibberish.

Likewise, Zodiac’s supposedly impressive cultural knowledge is overrated and juvenile. He quoted the Mikado, which led someone to refer to him as a "serial killer who enjoys light opera." In reality, Z didn’t know the stage version of the Mikado. He was quoting the Groucho Marx version. The "book" he quoted, The Most Dangerous Game, isn’t even a book. It’s a 15 page short story. He may not have even read it. It was turned into an episode of Gilligan’s Island two years before his letter. The inspiration for his masterpiece – the Halloween card – was a Tim Holt comic book. It doesn’t take a genius to enjoy comic books, Groucho or Gilligan.