Bloomberg had a video about this, one of their journalists wrote a book about him. The conclusion of that book was that Sarao didn't break the markets, and it was probably not necessary to throw the book at someone who was mentally disabled because some incompetent prosecutors needed a scapegoat. If you only read about Sarao within the context of the Flash crash, you will get the wrong idea.

Personally, I treat documentaries as entertainment. They can be informative, but I usually only pay attention to the finer details they provide proof for. Any kind of arch or narrative I remain skeptical on.