I have no problem judging other cultures or even imposing our culture's values on them through force, like we did with the Japanese and the Nazis. Recognizing that other cultures have different rules of morality doesn't mean that I'm not willing to defend our culture. I'm also aware that those cultures would have imposed their wills on us if they'd had the strength to do so. If the Nazis had come out on top, it would have been considered moral to murder Jews. In the real world, winners get to impose their moral code on the defeated.
One system can only be superior if it more truly adheres to an objective moral standard (which must come from God).
Any system that is on top will claim that it represents the highest moral standard. Some will claim their superiority comes from God, others base it on ideas of racial purity or the historical dialectic. There is no such thing as an objective moral standard because, at the end of the day, the winners write history.
Since in your system cultures decide what is moral, murdering Jews can't be wrong if the Nazi culture believes in it. Your morals are not superior to those of any other culture - all are equal in your system. You may be able to overpower other cultures, but this does not speak to the issue of whether murdering jews is wrong. You cannot say objectively that murdering jews is objectively wrong, you can only say that murdering jews is disagreeable to you or your culture and you won't stand for it. This is the textbook definition of the philosophy of "might makes right." (Stalin believed what you believe.) One problem with your philosophy is that it makes no distinction between power and goodness. There is no goodness or virtue or right or wrong in your philosophy as these would require a moral standard for their existence; and no inalienable rights - no right to liberty, no right to life, right to property - no self-evident rights. The culture dictates your rights - which is not only in direct contradiction with the Declaration of Independence of the United States, but such a believe is anti-intuitive and contrary to right reason. This philosophy is devoid of any humanity and is highly dangerous and destructive as the only way to adjudicate differences between cultures is WAR.
Another problem with your view is it does not take the individual into account - each person is a moral agent who can agree or disagree with the culture-at-large and make moral decisions against the cultural norms, so each person is his own culture because each person has a set of morals that may not agree with the politically correct - like me.
Any system that is on top will claim that it represents the highest moral standard. Some will claim their superiority comes from God, others base it on ideas of racial purity or the historical dialectic. There is no such thing as an objective moral standard because, at the end of the day, the winners write history.
Cultural morals have no force - man's rules are made to be broken. In practice, each person is his own moral agent in your system. So, your system doesn't jive with real life. You can't make a person believe in another person's set of morals or in a culture's set of morals. YOu can make them say they do, but you cannot control a man's conscience no matter what you do. So might makes right is pitifully inadequate to account for personal moral codes. Where does that sense of ought come from? Why do we have it?
You are a moral relativist by your own testimony. Since you are a moral relativist, you cannot say anyone is wrong about anything - in fact, the word "wrong" is meaningless. In fact, I could kick you in the knee and steal your stereo and I could not be wrong if I thought I was right becuase you have no moral standard to go by other than the culture's and the culture's rules are meaningless to me. What are you going to say, that "Stealing is illegal?" Stealing may be illegal, but legality does not equate to morality in your system (in fact, laws can't have any moral base in your system) - it's just a set of meaningless rules enforced by brute force. Psychologists have a label for people who make up their own moral rules - psychopath.
Indeed, Osama bin Laden was absolutely RIGHT in murdering 3000 people in your system of morality because he had the might to enforce his morality on those people in the twin towers. It wasn't a terrible tragedy, human life isn't worth much to a radical muslim, and since cultures decide what is right and wrong, he was right! No way around it in your system.
Your moral system has been weighed in the balance and found wanting.