That they convinced themselves that they were acting justly demonstrates the flaw in ethical relativism.
Or that we’re convinced ourselves that they were demonstrates a flaw in ours. My point stands that is there is some overarching global morality, we’d have no way of know whether our ethics or theirs are better aligned with it. Maybe we’re meant to be killing the weak and taking all their stuff.
The fact that you think that maybe we're meant to be killing the weak and taking all their stuff also demonstrates the flaw in ethical relativism. Justice is pretty simple and universal. Don't hurt people and don't mess with their stuff. This has been the standard throughout the ages.