In my opinion this is a great short story, where there is the ABC of a short story plus religious dogmas with references to a famous play written by William Shakespeare.
The contrast between the dealer and Markheim is immediate and powerful, this is configurable by the exchange of their discourses.
In the first pages of this tale, there is a reference, or better a similitude to Ebenezer Scrooge (Christmas Carol, Charles Dickens) regarding two key points, the emotional transformation after the supernatural visitations of Jacob Marley, and the ghosts of Christmases, past, present, and yet to come, and most importantly the lack of the Christmas spirit, a ploy for to introduce a fundamental Christian's dogma i.e. the equation that the money are the devil's excrement; this concept has been introduced inside the plot using the phrase at the beginning and at the end: The Stock Exchange.
In my opinion "the visitor" is the conscience of Markheim, that has been absorbed and corrupted by his madness due to his thirsty of money.
Thinking profoundly on this character, I would like to mention that there is a clear reference to a famous play written by William Shakespeare, i.e. Macbeth for instance the "Banquet Scene" and the emersion of his madness. The main cause for their homicides, is the epic conflict between their free will and their conscience, and their fear of being punished by the divine and terrestrial laws.
In conclusion Stevenson even in a short story is able to philosophising.
In this case I can assume that as readers we have been led to believe and eventually say that the finance is a sin.