Just over a month before the arrival of this correspondence, on September 12th 1987, Jose Santo Bugarin (26) and Rodolfo Alvaro (31) were traveling on King Road, San Jose, California when their car slammed into four children playing on the front lawn of a residence, killing three and injuring one. The driver then backed over the bodies and left the scene. Both were apprehended a short time later. "Bugarin was arrested on three counts of second-degree murder. Alvaro also was arrested on murder charges". This was covered extensively in the newspapers.
One distinguishing characteristic of his latest communication was the use of a "death machine" as a weapon for running over children on Halloween, which meant there was a high probability that the Zodiac Killer took this from recent news stories, despite having previously used this phrase to describe his claimed bomb when writing his lengthy November 9th 1969 correspondence.
On October 20th 1987 and October 21st 1987 (predominantly) a major news story was published throughout America about vehicle deaths. The Los Angeles Times wrote "Alarmed by a rising number of injuries and deaths, attorneys general in 23 states Monday asked the nation's largest manufacturer of all-terrain vehicles to meet with them to discuss ways of making their machines safer. The request was made in a letter to Honda under the auspices of the National Association of Attorneys General. Texas Attorney General Jim Mattox said "These vehicles are rolling death machines. There have been nearly 800 deaths and 300,000 injuries related to these machines since 1982". This quotation was parroted in virtually every newspaper I could find. The coverage continued with the statement "In what remains a largely unregulated pastime, all-terrain vehicles have been linked to an average of 20 deaths and 7,000 serious injuries each month nationwide in the last two years, according to a federal safety agency. About half the victims are under 16". The fact that the Zodiac Killer chose the phrase "death machine" while newspaper stories were abound with vehicular "death machines" being a major talking point, is a strong argument for the two being connected. Therefore it's not difficult to see how the Zodiac Killer may have arrived at the wording of "I will be out driving around on Halloween in my death machine looking for some kiddies to run over". But what inspired the Zodiac Killer to use "death machine" in 1969?