We as a society all follow pretty much the same moral code. There is not a significant portion of our society calling for a change in our morals that would allow for murder and theft.
(e.g. you don't believe me as to what is moral, because I adhere to Biblical standards of morality. Am I not part of our society?)
We don't share the exact same moral code, but we probably agree on 95% of what is wrong and right.
Who makes the determination as to which standards work?
History. We can look at the moral codes of various historic societies and see which ones worked and which ones didn't.
You suggest Rome? A great beginning, but in the end...disaster!
Wrong. A significant portion of our society says that killing a child before birth is not wrong. That is a violent change from the society that I grew up in, in the 1950's.
A significant portion of our society does not think that cheating on your income tax is wrong (theft), and the vast majority of students do not think that cheating on tests is wrong (also theft).
This culture has changed drastically for the worse in the last 40 years because of the abandonment of absolute moral principles.
(And sexual immorality, causing destruction of families and children, rampant disease, and even death, is one area where our collective 'society' says OK, do whatever you want, and standing idly by as the culture goes rapidly over a cliff).