What makes a decision good or bad is whether it would be the decision rational people would endorse in a perfect society.

I actually happen to think that human morality is a fit topic for empirical inquiry, same as human language. This is a wildly different approach from either the Kantian or the Rawlsian approach. To study English, we look at the actual practices and we (possibly) develop hypotheses about the development of English and of language in general. What we do not do - in an empirical study of English - is ask ourselves what grammar, what pronunciation, what meanings we would prefer in a perfect society. Such questions are what the creators of Esperanto asked themselves (I presume). Kant and Rawls are trying to do the moral equivalent of inventing Esperanto. I, in contrast, think that morality is something that, like English and French, already exists in the world, possibly varying a bit from place to place.

I realize that Kant and Rawls seek to critique our actual practices. It may seem puzzling for me to say so since I just explained my preferred approach as empirical, but so do I. But I do so from a different direction. Just as linguists will distinguish between natural language as it arises spontaneously among speakers, and the pedantic rules endorsed by language mavens, so do I distinguish between morality as it would arise spontaneously among people, and the laws raised over us by legislatures.