Actually these ethics, in a democracy are determined by the majority of people via their elected representatives.
Edit: I do agree with the sentiment of your post though. That is, to counter the idea that “ethics” depends on whatever view of the world an individual has.
I mean, not long ago, these same majority of people wouldn’t have voted for African americans to go to the same schools as white children.
And that’s all within this current generation
Hanging homosexuals is ethically wrong even if it fit popular support. Slavery is ethically wrong regardless of what people vote for.
How we judge past decisions is more complex then only the ethics, including knowledge, context, norms, etc.
but the ethics can be pretty clear
(can be because they can also not be clear, but they can certainly be clear.)