Any kid who grew up in Northern California in the late 1960s is going to remember the Zodiac killings. I was one of those kids, and can attest that there was something about them that was absolutely terrifying. The Manson Family murders were bad enough, but at least they caught the Manson Family. But when you're nine years old, and you're trying to go to sleep on a hot summer night with your window open, and you just know there's this crazy killer out there...well, that makes quite an impression.
The sections of Zodiac which depict the killings convey that terror with stunning clarity. There's no gore to speak of, and you know what's going to happen, but the sense of dread - the sense of why is this happening - is so stark, and so stunning that it threatens to overwhelm the entire film. It's a testament to director David Fincher that the film never flags, never lets up. The sense of mystery - the need to reach closure, as elusive as it may be - fuels the film, and makes everything that happens throughout riveting.
The actors and performances are uniformly outstanding - Jake Gyllenhall as the cartoonist who takes it upon himself to bring closure to the case and turns his quest into an obsession, Robert Downey Jr. as the dissolute but dedicated reporter who becomes Zodiac's pen-pal, Mark Ruffalo and Anthony Edwards as the detectives who are ultimately defeated by the case, and John Carroll Lynch as a Zodiac suspect.
But in the end, it's the sense of mystery, the obsession, and the terror that drive the movie, and make it work.