Below is a sketch of a moral anti-realist position based on the map-territory distinction, Hume and studies of psychopaths. Hopefully it is productive.

The Map is Not the Territory Reviewed

Consider the founding metaphor of Less Wrong: the map-territory distinction. Beliefs are to reality as maps are to territory. As the wiki says:

Since our predictions don't always come true, we need different words to describe the thingy that generates our predictions and the thingy that generates our experimental results. The first thingy is called "belief", the second thingy "reality".

Of course the map is not the territory.

Here is Albert Einstein making much the same analogy:

Physical concepts are free creations of the human mind and are not, however it may seem, uniquely determined by the external world. In our endeavor to understand reality we are somewhat like a man trying to understand the mechanism of a closed watch. He sees the face and the moving hands, even hears its ticking, but he has no way of opening the case. If he is ingenious he may form some picture of a mechanism which could be responsible for all the things he observes, but he may never be quite sure his picture is the only one which could explain his observations. He will never be able to compare his picture with the real mechanism and cannot even imagine the possibility or the meaning of such a comparison. But he certainly believes that, as his knowledge increases, his picture of reality will become simpler and simpler and will explain a wider and wider range of his sensuous impressions. He may also believe in the existence of the ideal limit of knowledge and that it is approached by the human mind. He may call this ideal limit the objective truth.

The above notions about beliefs involve pictorial analogs, but we can also imagine other ways the same information could be contained. If the ideal map is turned into a series of sentences we can define a 'fact' as any sentence in the ideal map (IM). The moral realist position can then be stated as follows:

Moral Realism: ∃x(x ⊂ IM) & (x = M)

In English: there is some set of sentences x such that all the sentences are part of the ideal map and x provides a complete account of morality.

Moral anti-realism simply negates the above.  ¬(∃x(x ⊂ IM) & (x = M)).