Peter Mcdougall from Scotland is a successful playwright in the 1970s. He also wrote scripts for both TV and film.
I think this is just one among the gazillions of scripts that has landed on Rob's lap. It doesn't mean he is gonna do it. With Rob's current Hollywood status, it is understandable why he is on top of everyone's priority list. Check out some info I gathered about the plot of this James Hogg novel. Pretty weird and a bit disturbing.
The Movie's plot. "Justified Sinner at its simplest contains the memoir of a young man, Robert Wringhim (or Wringham), who encounters a shape-shifting figure only ever identified as "Gil-Martin". This urbanely mephistophelian visitation — an early instance of the doppelganger in fiction — appears after Robert is declared by his "adopted" father to be one of the elect, and therefore a "soul" predestined to attain salvation. Although this invests Wringhim with a sense of infallible moral justification, he is at the same time tortured with self-doubt. Ostensibly co-erced by Gil-Martin (who the reader clearly sees exploiting these two mental states) Wringhim is led to commit a long series of offences, including multiple murders." Read more