This argument taken ad infinitum though devolves into a pit of moral relativism. I happen to completely agree with your argument—ethics can be derived as a functional application of rational logic. But I think the more germane point being made was that establishing the normative sociocultural definition of collective Ethics, is ultimately an emotional determination (if guided by rational logic). There exists no metaphysical Ethics which can always be axiomatically derived on the basis of rational logical. At least I think that was the argument anyways…

Yeah the crux is, if you invent your own ethics then how do you make sure that they are universally applicable? How do you make sure people will agree with you instead of creating mutually incompatible ethics with the same intentions as you? You base them on logic with a sprinkle of emotion.