"The era in which Zodiac takes place bridges two eras in urban America. The Zodiac appeared on the tail end of a crime-spree that rampaged across the US in the late 1960's. His settling in the SF Bay Area may be one of a number of social phenomenons that pushed America's view of itself out of an innocent 1950's sensibility and into a harder and darker view that became more prevalent starting in the 1970's and into the 1980's. People, even in urban areas, used to be far more trusting of one another, friendly, and civil. Many of the events of the 1960's gave urban Americans a much more cynical and cautious attitude toward people they didn't know. Don't trust or talk to strangers. Better to sacrifice helpfulness than to wind up dead. People are out to take advantage. At least in urban areas nowadays, it seems, people are much less willing to take the risk to meeting someone they don't know, largely out of fear"
...
"The Zodiac came to personify one of the constant fears of living in urban America: a faceless, emotionless killer that comes out of the shadows of a dark alley to commit heinous violence. In the end, we fear strangers because of this, but we end up sacrificing love. It is an ironic aspect of human nature that people can do to strangers what would be almost unthinkable to do to people that we know. In addition to the poor innocent people that were brutally murdered, the Zodiac committed another crime against humanity. He compromised our sense of trust, civility, and in many ways, love for our fellow human beings even when we might not know them."
--imdb
