I write a bunch of sensitive topics, psychological horror, and horror in general. I have very strong feelings on this matter;
When it comes to writing adult stories for adults... The only moral obligation I see is that 18+ material be marked as such. Anything more and you start going into dangerous "book burning" territory imo. What I've learned from taking my works to adult groups is that everyone takes something different away from your work. No matter how horrific I try and write a scene someone still may find it titillating. I do believe I should be able to write the prospective of a villain and trust my audience can see through the "justifications" the character so confidently spouts, however that is not always going to be the case. People may just take it at face value. I can send a short metaphorical story about a mental illness to someone who works in the field and get back how well I portrayed it and how accurate I had tackled it (not that I needed to since I was taking observations from my own life) and still get someone offended and accuse me not understanding and being tasteless. There is no way I am going to dumb down my work and type out the morals or whatever word for word because some adults lack the comprehension skills to read between the lines. I want to tell a story, sure, but I also want to inspire thought. Emotion. An experience. And the only sure fire way to make sure no one gets the wrong idea about your work is to take a step back and write an authors note explaining everything in a "Please I have the correct and moral way of thinking!" type letter. Media literacy is bad enough as it is, that being expected to assure people are "morally responsible" would be a poison, truly.
So. What of the common arguments I see; "it has to be tasteful" or written with "good intent" (which is a standard I do not agree with)... how can one tell? Who's opinion counts here? Who is the arbiter of what's morally correct to write or not? Where is the line? Is it where Billybob from the bible belt wants it to be-- blocking all LGBTQ content and calling abortions first degree murder so all works must be written with that in mind? Probably not. Is it where you, whoever is reading this, thinks it should be? Well. What makes your opinions more important that anyone else? Or do we simply expect every piece of work to be the author's opinions on the matter? Over all, this paragraph is worthless because it doesn't matter. No. It should not have to be written tastefully, whatever it may be. Did you know, one of the most common fantasies in both men and women is a very horrific crime? I don't know if these forums are okay with me saying the word, but it's probably one of the first 18+ taboo things you think of. Should that not be allowed to be explored in a titillating fashion because it's morally reprehensible? What would this extend to? Can we not write murders in any sort of satisfying way? A heist? Or... just the crimes you, whoever is reading, don't like?
Adults shouldn't need to be babied over such things. Sometimes adults want to explore awful taboo things in a safe environment and that's okay. Dark romance is a popular genre for a reason; and it's not because girlies think abusive relationships are so morally sound and desirable or whatever. Though many don't admit it, many people like dark and messed up stuff. It's interesting. You do just kind of have to trust your audience knows right from wrong at this point and can separate fiction from reality. After all, you're writing for adults, not teens nor kids. If someone chooses to act on some fictional thing you've written that is beyond you at that point and no, you shouldn't feel or be responsible for it.
TL;DR and Point of all that being, the idea of only writing things for the right reason or whatever is akin to censorship in my mind, and censorship is an absolute plague that rots away creativity. The only moral standard you're thinking of writing to is your own, which is your prerogative. That said, all that is why I do believe the moral obligation starts and ends at labeling such content. I can write whatever I wish, explore things that are awful morally in a safe and creative way, all that stuff WHILE readers can stay the heck away from it.
Sorry if this was just a bunch of rambling or doesn't answer your question...