The Zodiac Killer's response was swift, sending a letter to the San Francisco Examiner on August 4th 1969, replying directly to Jack Stiltz and stating "In answer to your asking for more details about the good times I have had in Vallejo, I shall be very happy to supply even more material". Without this prompt by Jack Stiltz, the pseudonym of "Zodiac" may never have come to pass - at least not by August 4th 1969.
The next communication was mailed just over two months later, on October 13th 1969, admitting to the murder of Paul Stine in Presidio Heights only two days earlier. But the Zodiac Killer hadn't forgotten that Vallejo Police Chief, Jack Stiltz had doubted his involvement in the attacks at Lake Herman Road and Blue Rock Springs, by stating he required "more facts to prove it". So, the Zodiac Killer in advance of the taxicab driver's murder, decided to remove a piece of the victim's shirt and leave law enforcement in no doubt he was the true killer at Presidio Heights. Two days later, he mailed the Stine letter, stating "I am the murderer of the taxi driver over by Washington St + Maple St last night, to prove this here is a blood stained piece of his shirt". The Zodiac Killer was responding to law enforcement who doubted him previously, but primarily to Jack Stiltz's comments in the newspapers back on August 2nd and August 3rd 1969.
Therefore, Vallejo Police Chief, Jack Stiltz was instrumental to the August 4th 1969 'Debut of Zodiac' letter in which the pseudonym "Zodiac" was revealed to the world, as well as the inspiration for the Zodiac Killer to decide in advance of his San Francisco murder, that something from the victim (a shirt piece in this instance} had to be mailed with his next correspondence, "to prove" undeniably he was the killer.