What does it mean when Death says "I am haunted by humans"?

"I am haunted by humans."

Death, the last line of the novel, p. 550

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.

Death's life is about seeing, experiencing, watching people die. Altogether, in their life, their living and their dying humans have an extraordinary capacity to do, to be and to become, things evil and things good. They are capable of becoming afraid and of acting courageously in spite of fear. It is this complexity of humans that makes death afraid, as it were, baffled and yet mesmerized by them. It is as if he/she would want to live and become friends with humans but at the same time he/she knows that he/she is one of their greatest enemies.

Additionally, I believe that sometimes death tries not to visit someone he/she loves until that someone has lived and has accomplished all their aspirations. In this scenario death becomes a voyeur, a student of the living.