Sarah: I didn't find the ending bothersome, or have those expectations for it, but I have read so much about unsolved cases that that's no longer an issue for me. There's a quote from John Douglas, the FBI profiler sensei, with regard to JonBenet Ramsay, where he says that every case has a bunch of misleading/irrelevant "evidence" that doesn't fit, and you have to learn to live with that. I always think of that quote when I watch the end of Zodiac.

But I disagree with Mike that the ending is that forthright"It could also be this guy...he had a face like this. No, sorry, it's the first guy I pointed at." Followed by a chyron saying Allen was not indicated by the DNA sample.

Mike: Ah, but he doesn't say "it could also be this guy." He's very clear at all times that it's Allen. He uses one of the other photos as a way of noting how Allen's face at the time of the incident differs from his driver's license photo. But he doesn't waver about the ID. At all.

Sarah: I don't see it that way. I think he was being influenced somewhat by Det. LeGros's eagerness for a firm ID. (I don't recall whether Mageau was this definitive in real life.)

What the film seems to want us to see in that moment is how badly the investigators, and by extension we, need a solid answer, something they can move on, whether that answer is "correct" or not. LeGros is practically vibrating. 

But your argument is solid. I'm not mad at it.

Mike: We see that very differently. (I just watched it again, twice, earlier today.) LeGros seems to me very concerned that Mageau will make a false positive ID. Tells him twice before they start that just because he's showing him a bunch of photos doesn't mean the killer is necessarily one of them.

Zodiac 5

And then when Mageau points to another photo (again, just by way of comparing faces—saying "his face was fatter then" would have conveyed the same information and been less confusing), LeGros asks "are you changing your identification to this person?"

I don't see him striving to steer Mageau back, or to necessarily get an ID at all. (Which is in keeping with the book, where the cop in question says he had no expectations at all and was just doing it to be thorough, which is why it happens in a freakin' airport.)

Sarah: I don't think he's steering him either, quite, but Mageau is not unaware of his importance as the only living person who saw the guy's face and how important this makes him to the investigation.