Mister Organ is one of the most unsettling documentaries I’ve ever seen
The detail that sticks with me is "intimate campaign of terror." The Act of Killing works because it's about a national nightmare, and Beware the Slenderman is about a crime that makes no sense. Mister Organ sounds scarier in a quieter way, because the threat is one man who just won't stop, aimed at people who did nothing more than cross him. That's the horror that feels possible in your own street.
The comparison to those other docs is smart framing. Farrier seems to know that scale isn't what makes something unsettling. A sustained, personal harassment campaign can be more draining to watch than a single atrocity because it has no endpoint, no catharsis.
What I'd want to know next is how the film handles the practical side: how long Farrier spent inside this orbit, whether Organ ever gets a chance to speak, and what the aftermath looks like for the people he targeted. If the film is as good as the description suggests, the power is probably in the accumulation, the small moments that add up to something unbearable.