Profile of the Zodiac Killer
Yet he may have grown bored over time. Like BTK, he seemed to have gone into a dormant period, perhaps several. The complicating factor is the copycat contagion. We can't tell which letters are definitely him or from someone muddying the waters, and we don't know if the Zodiac did in fact kill more people as he claimed to have done.
If we try to figure out his background - for example, if he had military training - we're faced with his diversionary tactics: the codes could have been bait that he picked up from a hobby or they could have been learned during a military stint. We have plenty of examples from him of meaningless directions, but we have no way to know which of his clues actually provides a means of identifying him.
I think he might have had minimal success at other things, at best. He might have had some kind of job or basic education, but in his own mind he was superior to those things, and since he couldn't milk much grandiosity from them, he turned to killing and bragging about it. He probably felt immortalized from all the publicity and from the books that had been written about him. If he's still alive today, he might get some satisfaction that, all this time later, his crimes can still command a huge international audience. Now he's really somebody — like Jack the Ripper.
The idea proposed that he might have known one of the victims and wanted to muddy the waters to keep people thinking her murder was a random stranger attack might be viable, but there's more pathology driving him than just covering up a crime. Some guys have callous disregard, making a crime look like a random act, but this guy was overtly aggressive. I don't think he was psychotic. If he were really crazy in a disorganized way, he'd probably have been caught, because he couldn't have kept it together.
Ultimately, I think, for Zodiac, these crimes were about getting notoriety. He immediately called them in, knowing he had gotten away and the police would likely come up empty-handed. That was as much of the thrill for him, perhaps more, than any other part of the incidents. It's like the case of the Tamiami Strangler in Florida; when the police didn't connect the victims, he wrote numbers on them. If one calls in one's own murders, he'll ensure that he'll get the credit. Zodiac might have lurked around the area to watch the response. That's the payoff. Watching the investigation would excite him. He might even chat up the cops at times, like Ed Kemper did. That way, he could be part of it, he could learn what the police knew, and he could enjoy how stymied they are. He'd go home and ponder that, enjoying it over and over to fuel his narcissism. Then he'd want to go out and do it again.
Even when he wasn't killing, he'd know that the letters would keep up the terror and excitement, so he got the same effect without having to take the risks. That makes it clear that fame and a sense of superiority were probably more exciting to him than the violence itself. He just wanted to be known as a notorious killer. It was probably the only thing he ever accomplished.