You could argue being ethical is to simply be logically equitable. It's ethical to treat others with respect and report bad behavior, sure, but it's also logical to buy into the standards of society in order to enjoy those same basic protections yourself that ethics offers to other people. Treating others how you want to be treated is a mutually beneficial arrangement.
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.
So does most sin. Do we need to be ethical if we are not evil?
Ethics helps us answer questions like "what do we owe to ourselves" and "what do we owe each other", which have nothing to do with sin, good, or evil.
Well the answer is usually something along the lines of, dont be evil to your neighbour (or some variation thereof, whether that's put in terms of utility or universal laws), so i think it is related.