For them it is. In Germany during the war what they did wasn't considered unjust (like in the South during legal slavery). And if the majority on earth agreed, it would be considered a-ok (like before countries started abolishing slavery, when it was just a normal thing for most peoples all over the world).

We do have some escape hatches from "it's just what most say it is", but those are based on religious and cultural ideas (including "scientific" conceptions of justice in the latter camp).

I think in the end: justice as an operative thing is what most people say it is (it can't be anything else). But there's also some common shared sense of justice, not well defined and not easily enforceable at all cases (e.g. one couldn't enforce it in WWII Germany), but which contains a core of the only universal notion of justice (don't cause harm without reason, don't kill, don't steal, etc).