Why is death telling the story?

Death represents humanity. He has know sides of emotional attachment to the human world. Death has no affinity for any "special interests." He notes that a big part of his job is equality, he comes for everybody sooner or later. This sense of universal fairness  makes him a reliable narrator. In this sense he is able to give us an objective, almost child-like view of humanity. He spends much time contemplating the heroism of people and the utter evil people are capable of. He serves as an honest outsider to the paradox of human nature.