I feel the few around him may have had suspicions at the time, but they either refused to or were afraid to come forward.
The Zodiac persona would be the crowning achievement in his life.
He was most likely a social oddball, suffering from paranoia possibly bipolar prone to severe changes in mood, depressed loner, few if any friends and most likely preferred it that way primarily during the period he was actively committing the murders
He would have lifelong issues with (which) gender he preferred sexually, domineering mother figure whom he probably lived with at least most of his life possibly during his crime spree, there appears to be an absent or abusive father figure, narcissism triggered by a severe feeling of inadequacy.
As is common with paranoia, would feel a overwhelming need to seek retribution for real or perceived, wrongs, however being a coward it would usually be nothing like a direct assault, damaging property, killing a pet, in this instance he would be more like a bomber or poisoner (hence the Unabomb references)
Meticulous at times, but stress and mood changes, would often cause him to fray and become sloppy.
Narcissistic traits, would carry over into most facets of his life, where he'd be known to be self absorbed, and think he was better than those around him, however it would be more muted than you would expect, most likely not well liked in the workplace. He would feel a need to demonstrate his superiority, often felt underappreciated or repressed at work. They are often a stickler for rules, and will readily point out anything their co workers do wrong , this feeds their superiority complex .
If any military service, would have had issues with discipline, behavioral or even health issues most likely leading to a discharge.
Firearms would be what gave him power power so he most likely owned several, these types will often be hunters etc..
The murders themselves, speak more to the true personality of Zodiac, the letters help to shape his image of himself better, to bring about a clearer picture behaviorally
The attacks were solitary attacks in dark areas where couples, were engaged, in sexual activity, or appeared to be so, he most likely staked out these areas prior, this isn't an individual to leave much to chance . This was viewed as a "wrong" by him, he was going to make those pay, for their wrongs, because it was what HE desired but was not able to have.
Some have labeled him a sadist, im not certain I would label him that, i feel the hood and outfit were more a part of the persona he created than seeking pleasure from the fear he caused, though he most certainly reveled in it, sadists derive arousal and pleasure from pain and fear this, this takes time and usually a location where the victim wont be discovered, though these crimes, were both sexual and terrifying seemed more to fill his narcissistic need for power.
The communiques were the second component, they were what he wanted us to see, HIS version of why and to scare the public.
Where we get the most, is the murder of the cab driver, which was out of line with his normal MO, this was in response to tha papers referring to him as a coward who preyed upon women. This demonstrates a need for HIM to dictate how he is perceived, and to re-establish his "machismo" .
So what happened to him ?
Experts feel initially he stopped, because he felt LE was too close, as in most cases, they may have even interviewed him, but for some reason they never tied him to anything.
AS we now know serial killers can and do stop for any variety of reasons unlike the previous notion that they only ever stopped because they were dead or in prison.
So why did he never re-appear?
Theories abound, but one I feel holds the most credibility , is probably the least interesting, when a paranoid , depressive, with severe mood swings no support system, begins to feels like they've lost the control they once held so powerfully, they are very prone to suicide.
I do not believe he went on to commit further crimes anywhere , i don't think he changed MO and kept on killing, I don't believe he later mailed bombs to executives. i feel that his only way to maintain power and control while dealing with severe depression, was to take his own life