Folks like to bring up the factual errors and omissions in Zodiac but it was never intended to be a history or expose of the killer. The core of the story is exactly what you described. It's about obsession. Where we choose to focus our intentions, and the consequences of neglect. ALA is our big suspect in Zodiac because that's who Graysmith and those guys spent years following. It's about them. John Carol Lynch is more or less a mcguffin in this movie, and his performance reflects that.

The movie does attempt to present events in a factual and methodical way, but these beats give us more of an insight into the days, months, and years our main characters were spending on this. And yes, a significant portion of the movie is used to recreate the murders as factually as possible, but I believe Fincher was just doing his due diligence whenever the facts were available. We don't see the murders because the movie is about the Zodiac killings, we see them because a) they're compelling and b) they give us a sense of the looming dread cast over that area in that time. Specifically over our main characters. There's usually some kind of audio that carries over from the end of the killing scenes to the next, suggesting this kind of omnipresent connection to their lives. Haunting and taunting them, especially Graysmith, while their lives slip by.

Then one day there he is. Robert gets his wish. He sees an old sad man in a store. And JCL gives Gyllenhaal that shit eating look. It's so perfect lol.