Why does Death tell Liesel that it is "haunted" by humans?

The Book Thief is framed by Death's contemplation of the worth of humanity, and Death's inability to reconcile the remarkable cruelty and the remarkable compassion of which human beings are simultaneously capable. Liesel's life story contains elements of both, and by the end of the novel, Death appears to be no more capable of judging humanity than at the novel's outset. Thus, Death tells Liesel that it is "haunted" by humans, just as humans are haunted by Death. A jaded metaphysical being so used to dying could only be fearful of -- and, at times, amazed by -- those who live.