You've died: early, suddenly, before you could atone for your sins. You're not the worst person, but the mistakes you've committed during your lifetime are enough to get you stuck in Purgatory for eternity. So you hang on to the only thread - you agree to become a servant and ambassador of the Death itself. The deal is simple: you get a portfolio on the next victim and take their soul by any means possible, or go to hell. (The black wings and a huge scythe are provided by the agency.)