My Work Is Not Yet Done by Thomas Ligotti

When junior manager Frank Dominio is suddenly demoted and then sacked it seems there was more than a grain of truth to his persecution fantasies. But as he prepares to even the score with those responsible for his demise, he unwittingly finds an ally in a dark and malevolent force that grants him supernatural powers. Frank takes his revenge in the most ghastly ways imaginable - but there will be a terrible price to pay once his work is done.
Destined to be a cult classic, this tale of corporate horror and demonic retribution will strike a chord with anyone who has ever been disgruntled at work.