Documentaries that pretend to objectivity are usually that, pretend. A point of view is there whether or not the film makers are open about it. Attica has a point of view: that the inmates had righteous grievances, and that the state (personified by Nelson Rockefeller) brutally and murderously shut it down. If you don't accept this, then you will likely have problems with Attica.
Stanley Nelson compiles footage from the event, adds current interviews with some of the participants who are still alive, and tells a compelling and infuriating story. The basic facts are there ... prisoners rebel, take hostages, make demands, observers enter the picture (many of them well-known), change seems possible, and then the whip comes down. People are dead, people are tortured, all at the hands of the state. When it is found out that several of the hostages died, the state claims their throats were cut by inmates. Which stands until the medical examiner says no one's throat was slashed, that they all died of shootings by the authorities.
Nelson presents an air-tight case. The problem, as is often the case with documentaries, is that what we see is selective. Nelson has a point to make, and he chooses what to show us to help make his point. This isn't exactly false, except by omission. There are a lot of questions to be asked, if you think beyond the film itself. Nelson's work is so powerful that those questions don't necessarily jump to the surface. I was convinced, but then, I came to the film already assuming the prison authorities and politicians up to and including Rockefeller were corrupt. (Nelson adds Nixon's name to the list of infamy.) I believed what I saw, and after the fact, I still believe. But this is not a perfect movie just because I agree with it.