Spoiler alert, read the story first!
This is an extremely demoralizing piece, because it is a parable of a person who is trying to do everything right, but is destroyed by unknown circumstances into doing things that are repugnant to any moral sense. It is essentially Job, except without the redemption, with a much more saintly character than Job could be, because her goodness is made manifest, not just asserted.
It produces a sense in the reader that one aught to do what is right, not because of any rewards, but intrinsically because it is the right thing to do. This is a very difficult realization to produce in a printed work, and Sade does it masterfully. It is a different approach to the prolbem than the direct approach, which he developed in the 120 Days of Sodom, but it is also effective. But this story is different from the 120 Days, because it demands that one suspend judgement on Eugenie, despite the damning facts. Facts be damned, it wasn't her fault!
The story is also extremely well plotted, very emotionally devastating, it is a work of horror with no zombies, no vampires, nothing, just human beings behaving in bad and not so bad ways, so that even a saintly character is placed in the position of participating in horrible evil, despite having no bad intention in her body.
It produces a different sense of right and wrong than standard religion, which would traditionally condemn Eugenie, despite her saintly nature. Instead, you are wounded by what is happening, you believe Eugenie is a saint! Yet still, look at what she does, through no fault of her own. It's a very difficult situation, which aims to reduce the judgemental aspects of religion, to empahsize on the desire to make goodness in the world, without judgement of others.
Lars Von Trier explored similar themes in his trilogy of films, beginning with "The Idiots", and culminating in "Dancer in the Dark".
You're thinking of Lars von Trier, not Gus Van Sant.
Oops! Thank you. Those Europeans with their names!