Why is it that most modern books and movies, when showing traits of the good character of an 18th century story, depict niceness in general with the assumption that they were nice to their slaves and helped fight for their freedom.

Are those the only kind of nice/good/honest/truthful people who lived in those times? And were all good people anti-slavery and all bad people pro-slavery?

By this definition, no plantation owner or anyone in his family could be a good person! Unless you let go of all slaves and became a pauper (imagine trying to compete against other plantations that had relatively free labour vs. “paying” for labour)

Being nice to slaves could not be, by itself, the only or the best criteria. Could it?

Thats like future books/movies depicting all nice/good/honest/truthful people from the 21st century as someone who never had a maid or always gave to charity or helped solve world hunger and fought against genocide by going to Darfur.
I havent helped solve world hunger or always given to charity. Heck I even have multiple maids at home. And I don’t see myself stopping this lifestyle right away.

Would I be portrayed as a bad person in future history?