On relativism:

In school we had a standard joke: "Everything is relativ!" meant: everything is equaly unimportant, nothing counts.
But, relativ means related. This means, it is connected within relations of more or less bigger contexts.
Coherence is the main term.
Taking Hitler as an example: he isolated his moral from the coherences of international agreements.
Relations and coherences can be discussed and lead to agreements, and people can base decisions and agreements and laws on it. And the value of these agreements raise with the respect of the participants who acts in accordance with these agreements. And the value of such agreements decrease with each violation which happen without having consequences for the violator.

In short, such "relativism" is the base of peace, the base of human rights etc.

And, it is a better base than any eternal instance of any group of the affected people, because it leads to balance of interests and powers although not everybody shares the same believes...