why do you think the author has "Death" narrate this tale, using chatty somwhat ironic tone?

Death is meant to be an ironic dis-attached entity. It is ironic that Death himself should narrate one of humanity's lowest moments, Death is working overtime. Normally silent death becomes chatty and ironic to hide his conflicting ideas (even emotions) about humanity. He is constantly confounded at the human capacity for heroics and sickness. Even death needs an outlet to make sense of them.