All these democracies, fascisms, communisms are not evil, not negative, but counterwise, they are ideals, goals and solutions how to make humans happy. Unfortunately, human nature is flawed, and flawed seriously, and no one counts these flaws. Democracy, fascism and communism become negative, only because peoples are negative. There are no universally constant values, and no constant evils. Just conflicts of interests of different peoples. Collisions of powers. And in these conflicts winners are always right.
This slavery, that you took as an example, is a negative association only because it is not acceptible in our society, no more and no less. But in different time and place, back in ancient Roman Empire, slavery was a common state of affairs, without negative and positive associations. And if you will take a closer look, even from modern humanitarian point of view, to even more ancient times, you will see, that slavery, actually, was a positive development: it saved numerous lives.
Everything is relative and we should try to abstract from modern and common points of view when we roleplay different universe, that missed many events of our history. There weren't any Hitlers, Mussolini, Trumans, Bushes, Stalins and other known to us peoples who committed crimes against humanity.
We must fight numbness of our own minds, and overcome ourselves to try to think in different categories, in different planes and universes, trying to analyze the situation from above, like a bird flying over a horde of mice.
But this is not easy, and many peoples are not ready for this, it needs more development, more intelligence, more self-control, more studying, and simply thinking (or meditating on a subject), it requires hard work. And by speaking with characters, we should make this easier, not harder!
Again, your example of slavery, IMHO is a bad example. You see, it is not just a word, it is a concept, and there are many different words for slavery in different languages. You may try to find synonyms, like "owning of peoples", but concept and association remains.
And fascism is a whole different story. The word gained negative associations, because fascists were associated with nazi, and because in some countries they were associated as ideological enemies of communists (and thus enemies of the peoples), they are associated with peoples who commited war crimes and crimes against humanity even without majority of them being fascists.
And, moreover, unlike slavery, there are many ways of describing system without using the word 'fascism', and putting such strong negative bias, that is very wrong.
I will give you an easy example. Imagine yourself building a dream society and starting to tell auditory about its properties. Someone stands up and says: "But this is fascism!" You have to admit, that it is fascism, but you thing about properties, and everyone in auditory think about crimes. They all shout to you, that it is bad.
Can you prove otherwise?
I don't think so, because they won't hear you, because they won't understand you.
It is a stalemate, when peoples think about different things and name it with one word. This is an ice wall that is very hard to break. And it is in our mutual interests to avoid such situations.