What is the writer's goal in Bartleby the Scrivener, A Tale of Wall Street?
What is the purpose of the story? Was it to raise questions, motivate people to take action, change readers' perceptions, or to merely entertain?
If we consider the narrator's sorrowful last speech in Bartleby the Scrivener--"Ah, Bartleby! Ah, humanity!"--then we are drawn to at least one conclusion about the story's purpose: that is, to examine the human condition. Although it is almost always difficult, and sometimes impossible, to determine an author's purpose in writing a particular piece, it is reasonable to believe that Melville--as he did in Moby Dick--used the story of Bartleby to comment on man's condition. In this case, although the narrator seems to conclude that man travels through life essentially alone, the fact that he even comes to that conclusion is, from Melville's perspective, a good outcome.
Despite the story's title, this tale is about the narrator rather than Bartleby. More important, the story chronicles the narrator's growing awareness of and sympathy for the difficulties inherent in human existence. For example, at the beginning of the story, the narrator describes himself as
. . . a man who, from his youth upwards, has been filled with a profound conviction that the easiest way of life is the best.
Throughout the first half of the story, the narrator remains completely detached--a very cool observer--from the lives of those around him, including Bartleby. As his experience with Bartleby increases, though, the narrator begins, for the first time in his life, to feel some sympathy for Bartleby. After he realizes that Bartleby has been living in the office, apparently eating nothing but ginger nuts, the narrator has an epiphany:
Immediately then the thought came sweeping across me, what miserable friendlessness and loneliness are here revealed!
This realization constitutes the beginning of the narrator's own true humanity, which continues to grow as he wrestles with his attempts to help Bartleby, which include taking Bartleby into his own household--all of which Bartleby rejects.
Bartleby then becomes the catalyst of the narrator's spiritual growth. Before his experience with Bartleby, the narrator says that he had never experienced anything but "a not unpleasing sadness." His experience with Bartleby then engenders an understanding of "a common humanity." The narrator has, in effect, become a fully-rounded human being at this point, the only character in the story who represents spiritual and moral growth.
In essence, then, it's likely that Melville is exploring both the human condition and how that condition can be altered by experience and human sympathy. To answer part of your question, I think that this story explores human nature and argues that nature cannot be changed but the human response to that nature can be altered.
In Herman Melville's case, we know that he was interested in exploring the human impulse to make decisions, and the ratio of human action to cosmic or natural forces in the outcomes of earthly events. Bartleby, in Bartleby the Scrivener, A Tale of Wall Street, is an extreme example of not making conscious, consequential decisions (except the decision to not to make any decision); the comparison with the decisions made in Billy Budd or Moby Dick--two extreme plots driven by human decisions--underlines how Melville was very interested in that ratio and its balance.