The Corpse Thieves by Charlotte E. English

Konrad Savast is the Malykant: foremost and most secret servant of the God of Death. His job? To track down the foulest of murderers and bring them to The Malykt's Justice. No mercy. No quarter.

Danil Dubin is a murderer. He knows this because he's been told — by the many witnesses to his crime.

Only, he has no memory of it himself.

It's part of a spate of similar killings, and Konrad must act. But how can he deliver The Malykt's justice to a man with no reason to kill, and no knowledge of his crime?