What if most of the ethics or morality we know in philosophy or common sense ethical reasoning are seriously shaped by our building of fictions based on ethical intuitions? That's not to say that necessarily these intuitions are wrong (error theory on these intuitions). Even if they are right these intuitions may be based on one or a few paradigm cases and though have some truth in them, small inaccuracies in the way morality is built up from them may magnify and cause ever more fictions to be built on top of earlier formulations. These fictions may in turn cause frictions with either each other or other aspects of what we know. That's not to say that harmonization of these fictions with each other and other aspects of our knowledge are not possible, it's just to say that any harmonization may be in some sense arbitrary and based on fictions.
As one example could it be that our intuitions regarding moral responsibility and especially contempt and its opposite, praise of moral character, are based on fictions we have of each others characters such as that these character traits are relatively permanent (maybe even across possible worlds) and rather consistent with other character traits of the individual? We may have invented conceptions of personhood as enbodying relatively permanent and consistent character traits based on justifying our practices of attributing blame and contempt, eg such as fictions regarding people's character or personality. It may also be based on the fiction of alternative possibilities assuming that it is a fiction (my point is that we don't know if determinism is true and so my calling it an invented fiction may be justified on that epistemic lacuna). As we know more about the world, we would have to come to ever more elaborate fictions to justify our practices or else risk not being able to justify them as they conflict either with each other or with other notions. If determinism is true, we may have to change our conceptions of what it means to be a person (i.e., come up with further fictions which in turn may or may not come into conflict with either other moral intuitions or what we will learn in the future about the world).
Let's say that one day we were to learn that the Nazis are very much like us. That is, that had most Nazis been raised in different environments like ours or we had been raised in theirs, they would have turned out very differently and not been subject to the contempt we have of them and may even be the objects of praise. Consider Hitler; it's possible that had he been raised differently, he might have turned out a good person. We may have invented a character type for him which is false and his fellow Nazis. Since many of our reasoning may be based on using paradigm cases of evil and these are our paradigms, many of our reasoning may be contaminated in a sense.
This is analogous to the fictions we sometimes invent (as the story goes) in mathematics such as inventing the fiction of sets, etc. Now it may the true that some of our “fictions” may actually turn out true in which case they would not be fictions but it seems that they could as well turn out to be fictions and the long history of ad hoc rationalizing and creating of fictions needed to form coherent pictures of either morality or mathematics may count against them as such. If they turn out true, that would be coincidental.
However, there may be moral facts out there. It's just that they may not square well at all with our moral common sense or reasoning as these maybe the products of our fiction building as well as our intuitions. Moral facts and a truthful moral theory may be far more nuanced and unrecognizable from all of our notions.