Rational ethics are just game theory applied to identifying symmetric positive sum rules for a group of N individuals, where N gets large.
They are not generally arbitrary, being the product of math and self-interest.
Where there are multiple equally good alternatives, the choice between them could be arbitrary.
Also, I don't think ethics is a local maxima that can be found through optimization. Basically, it's not an absolute truth of the universe, but a set of arbitrary rules invented by human. I think it's much closer to a chaotic system - which can radically change in value even by a slightest change in the underlying parameters, but is still governed by a set of simple rules. Thus, we would need more symbolically capable systems to process contexts based on the rules of ethics, and we're currently far away from this AFAIK.