From the law. The public law (constitutional law, administrative law and penal law) have objective definition of good and evil.
Which country's law is the one true morality?
The preambule of the American Constitution speaks of justice, general welfare (common good).
So the common good is a constitutional axiom.
Nothing is argued over more here than which
"common good" is the correct one. Even if we
here all agreed, this doesn't mean that our "good"
is the same as another country's "good".
But the reality is that both in Europe and in the US, the financial élites outrage the Constitution every day with their own selfish actions.
Thar is why the very same élites fund the Frankfurt School ideologies in any university.
So they can vilify the definition of good and evil, which are present in our constitutions.
I say that morality, ethics, good, & evil are human constructs.
They're like religion, ie, people feel strongly like objective
truths to some people, but others, especially we atheists,
see morality as a product of genetics & social interaction.
What we call "good" & "bad" are merely widely accepted
values. And those vary greatly with time & location.
I have values about right & wrong, but they're just my opinion.
One can reason from such premises, but one's conclusions
don't necessarily comport with beliefs of other people.
Rights to abortion, guns, speech, capitalism, religion, etc
are all in vigorous dispute.