The Private Memoirs and Confessions of a Justified Sinner by James Hogg

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.