Though there is a geographical relationship, and some vague similarities, I don't believe this is related to the Zodiac case,
It apparently started out as a foiled burglary, the Suspect, fled and was on the run, its unknown whether or not he had a weapon prior to stealing the car, however, when he attempted to rob the couple, he didn't have his own weapon, the 410 just happened to be in the car when it was stolen.
It seems most plausible , he came upon the couple by sheer chance,
The murder was a felony murder (murder committed, during commission of a felony), basically a robbery gone awry, its possible he didn't even intend to fire the weapon but panicked when the driver, gunned the gas, or even did so accidentally. Its also possible he was under the influence, of drugs or alcohol.
Which is the angle you would take when interviewing the suspect
This doesn't appear to be a sexual homicide, (which is what the Zodiac crimes, as were the Son Of Sam crimes, classified as, even though there was no sexual assault in those particular cases)
Look for burglaries, in the area, it seems unlikely he would simply select a random home in the area to burglarize, unless he was high, without trying others in the area 1st. Robberies, would also be a possible lead.
Its possible the burglaries, and robberies, are a means to feed, a dependency.