Reapers of Eden by Zero

Former math teacher Alexander Kuros has found himself better qualified for a new job: the Grim Reaper. Qualification number one for being a math teacher? You must be alive. Qualification number one for being the Grim Reaper? You must be dead.

After dying in a car crash, he is resurrected by Death as the new Grim Reaper and given the name Alpha. But when Alpha learns that the role of his new job is to kill people, can he overcome his humanity, or will he spend his new life living in guilt? And when he discovers that there is another reaper who wants nothing more than to send him back to the grave, he must come to terms with himself as the Grim Reaper or risk dying permanently.