Actually it is not just Zodiac who wrote or pissed with the cops, The poisoner Graham Young is another, I am sure there are more.
Is it not true that 'most' serial killers are of above average intelligence?
Seems to me, that if the killer is not a psycopath, then they might be into playing with the police/newspapers in an effort to prove to themselves, and the avid readers, how clever they are.
Reading their name in the rags and a print out of their clue/s.
I don't know much about Zodiac, but I'm sure I read that his clues were partly solved or at least the code was broken. In the mean time, what fun he had.
Regarding the first post, I think I have only read one book that connects Druitt, Stephen and royalty, and that is of course around JM.