I think his letters should always be taken with a grain of salt. He was probably just building his own myth, the article discusses that:

> As with most Zodiac writings, it sounds more like myth-building than sincerely held beliefs. Zodiac plagiarized often from pop culture, stealing ideas and lines from movies, operas and short stories. His letters, even more than his actual murders, were carefully constructed to present himself as the ultimate supervillain.