The narrator, an elderly, unnamed Manhattan lawyer with a comfortable business, already employs two scriveners to copy legal documents by hand, Nippers and Turkey. An increase in business leads him to advertise for a third, and he hires the forlorn-looking Bartleby in the hope that his calmness will soothe the irascible temperaments of the other two.
At first, Bartleby produces a large volume of high-quality work. But one day, when asked to help proofread a document, Bartleby answers with what soon becomes his perpetual response to every request: "I would prefer not to." To the dismay of the lawyer and to the irritation of the other employees, Bartleby performs fewer and fewer tasks, and eventually none. The narrator makes several futile attempts to reason with him and to learn something about him; and when he stops by the office unexpectedly, he discovers that Bartleby has started living there.
Tension builds as business associates wonder why Bartleby is always there. Sensing the threat to his reputation but emotionally unable to evict Bartleby, the narrator moves his business out. Soon the new tenants come to ask for help: Bartleby still will not leaveâhe now sits on the stairs all day and sleeps in the building's doorway. The narrator visits him and attempts to reason with him, and surprises even himself by inviting Bartleby to come live with him. But Bartleby "would prefer not to." Later the narrator returns to find that Bartleby has been forcibly removed and imprisoned in the Tombs. The narrator visits him. Finding Bartleby glummer than usual, he bribes a turnkey to make sure Bartleby gets enough food. But when he returns a few days later Bartleby has died of starvation, having preferred not to eat.
Some time afterward, the narrator hears a rumor that Bartleby had worked in a dead letter office, and reflects that dead letters would have made anyone of Bartleby's temperament sink into an even darker gloom. The story closes with the narrator's resigned and pained sigh, "Ah Bartleby! Ah humanity!"

Question: What is the moral of the mentioned dead letters?
Answer: The moral is they lead someone to go to a darker place. 
Alternative answer: It would make anyone like Bartleby more depressed