I don’t think you have to postulate Space Cannibals in order to imagine rational creatures who don’t think murder is wrong. For a recent example, consider Rwanda 1994.
And I think it’s quite possible that there might exist moral facts which humans are incapable of perceiving. We aren’t just universal Turing machines, after all. Billions of years of evolution might produce creatures with moral blind spots, anologous to the blind spot in the human eye. Just as the squid’s eye has no blind spot, a different evolutionary path might produce creatures with a greater or lesser innate capacity to perceive goodness than ourselves.
A duty is half of a contract—it comes from some obligation assumed (perhaps implicitly) in the past. A man may in general assign a very high priority to keeping his promises. He may feel a moral obligation to do so, independent of the specific nature of the promise. Should keeping a promise be difficult or unpleasant, he will balance his desire to avoid unpleasantness with his desire to be the sort of person who repays what was given.
For example, a man who has enjoyed the rights and privileges of a citizen may feel he has a duty to support the interests of his country. Certainly many citizens of the various States felt so, two hundred and thirty-two years ago.