What is the main conflict in Reaper Man by Terry Pratchett?

In Reaper Man, Death develops a personality and begins a mortal life as farmhand Bill Door, starting a backlash of unnatural events including an undead wizard, poltergeist activity, and animated objects. Worst of all, a dangerous hive mall is hatched from "city eggs." Ultimately, the mall is fought and destroyed. Through forging friendships in the mortal realm, Bill Door determines to fight Death, and he defeats the new Death and again takes on the role.