Out of all the comments I've made regarding morality, which is a lot, I've never been presented with a case as to why their morality is true/correct. Not once. They don't defend their presuppositions, they ignore them. When I ask how can you determine which person is right when someone within the same worldview conflicts with your morality, they ignore it.
The case that's been presented to you many times is simply that morality is subjective. What one person believes to be immoral may not be the same as what another person believes to be immoral, it's as simple as that.
Your argument seems to be that since all standards of morality are therefore equally valid it's impossible for any individual or group to judge the actions of any other individual or group, because to do so would be to proclaim that their standard of morality is wrong, and if someone truly believes in subjective morality then such a conclusion simply isn't possible.
Therefore subjective morality can hold no authority over anyone but oneself.
But I would argue that your reasoning is flawed, because it misunderstands the nature of morality. Morality is simply an evolutionary trait, wherein the standards of behavior that best ensure the stability of society will inevitably be the ones that survive. In some sense this is good, and in some sense it's bad. It's good in that morality has evolved to embrace justice, and empathy, and the golden rule. And it's bad in that it tends to cling to the intolerances and prejudices of the past, wrapped up in the guise of gods, divine laws, and cultural taboos.
Morality is the way it is because it evolved that way. It's what you get when you take an individual's sense of right and wrong and filter it through the brutal lens of survival. On the one hand you get "
an eye for an eye", and on the other hand you get "
the golden rule". For better or worse, morality is part of us. It shaped us, just as much as we shaped it. Perhaps we needed a God, or a book, or an institution, to inspire and defend our better traits, but we have to be able to accept them for what they are, they're the hallmarks of our past, of where we as humans came from, and the better part of what we are.
Morality is more than just each person's subjective interpretation of right and wrong, or some objective writ sent down by God. It's right and wrong refined by our struggle to survive. At its core it's still subjective, but to think of it as simplistically as that, is to diminish the struggle that instilled it in us.