How can I explain it without innecesary controversies in a soft way?
OK, let's try it with other example, "7th Sea", a TTRPG that drinks as source of inspiration a lot about the History of Western civilitation. History can be a fabulous source of inspiration for your games but there are some serious danger. You have listened "History is written by the winners", and this is true but also we should add "and after it is rewritten by the current rulers". Everybody says to be the good guys and the others are the evil empire. This shouldn't be a surprise for any body, it is not when victories are remembered and humilliating defeats are forgotten, but also the propaganda keep telling "the world before us was a hell, a dark age, until we arrive and took control, then we fixed everything, you're wellcome". To make to shine their own glory the ones by the rivals have to be hidden. Take care because the History can be full of half-truths, omiting some very important details. "7th Sea" could be a good example of how "History is rewritten by the current rulers". You have to learn to realise when fiction can be used for propaganda or to promote potentially dangerous ideological prejudices.
If you are reading/watching fiction and you suspect that character is evil, will betray the heroes or cause troubles because he belongs certain stereotype, then an ideological prejudice is being promoting. It is not only annoying but potentially dangerous. Don't be naive, the entertaiment industry is used for propaganda, and lots of consumers are noticing and they stop to buy.
We have to know the History, to learn from the past so as not to make the same mistakes. We need humility and self-criticism to avoid becoming fanatical monsters. Friedich Nietsche said: "Whoever fights monsters should see to it that in the process he does not become a monster" but I also I warn you: Don't allow shame and guilty-feeling were used to manipulate you emotionally. Recognize toxic people's danger signs before you get fooled.
Why are we too used to watch in the fiction certain group like the "bad guys" or "evil empire" but saying the same about other is "hate-speech"? Did you know the famous pirate Barbarrosa was Otoman (with Greek origin?).