The last words of “Bartleby the Scrivener” can be seen as a lament for the sheer absurdity of Bartleby's existence. They come after the lawyer narrator, Bartleby's former employer, has heard a rumor that the troubled young man once worked at the dead letter section of the post office.

Dead letters are those belonging to people who are now dead, and, as they cannot be delivered, need to be destroyed in order to protect their contents. The lawyer seems to think that the piles and piles of dead letters, and the absurdity that they represent—after all, letters that cannot be delivered are absurd—somehow must have unhinged Bartleby, leading to his odd behavior.

The final words of the story can also be seen as an expression of exasperation on the lawyer's part at the absurdity of life as a whole, not just Bartleby's life. That's why the lawyer exclaims “Ah, Humanity!” He clearly thinks there's something absurd about human existence, and this makes him a kind of proto-existentialist.

Little wonder, then, that Melville was such a favorite of Albert Camus, the famous French existentialist writer of the 20th century. In the figure of Bartleby, he clearly saw the prototype for the existentialist heroes of his own fiction, struggling vainly to survive in a world that is fundamentally absurd.

In "Bartleby the Scrivner" what might Melville have meant by the line "Ah, Bartleby! Ah, Humanity!"

This quotation comes at the end of an addendum the lawyer supposedly added to the story. In it, he tells of a rumor that Bartelby had been fired from a job at the dead letter department in the Post Office. Evidently, it was his job to sort through letters that could not be delivered. As he would sort through the letters just before they were burned, he would often find things like rings, or money desperately needed by someone who never received it. The constant shuffling of the letters which would never reach their intended destination, along with some of the items contained in those letters, eventually made Bartelby very depressed. That depression ultimately lead to his death. At the end of the story, the lawyer seems to offer a prayer for Bartelby and those like him who are forced to do thankless jobs day and day and eventually go mad because of their job. He says, 

". . .pardon for those who died despairing; hope for those

who died unhoping; good tidings for those who died stifled by unrelieved calamities. On errands of life, these letters speed to death.

 

Ah Bartleby! Ah humanity!"

The lawyer seems to be asking for some understanding for Bartelby as well as for those among humanity who suffer the same fate. Thus, this is Melville's way of making Bartelby's story more universal. Using the lawyer as his voice, he points out that other often suffer the same fate as Bartelby and like, Bartelby, die unnoticed and unmourned.