I read a nonfiction book about Matthew Hopkins, lo these many years ago as an undergraduate, which posits the same "general chaos of the English Civil War" theory for his success. IIRC, on top of everything else, the war left a power vacuum in that area, which made it relatively easy for Hopkins to jump in and do his thing: anyone who could have effectively stopped him was busy running about fighting the war.

Yellow Jack sounds dreadful. Why do dudes write such creepy books? (I know, I know, #notallmen, but all the same there's a particular type of creepy that almost always seems to be associated with a male author.)