Slavery is unacceptable whether or not the slaver is being nice to the slave.
Slavery disallows the productive futures of many people in society.
I'm sorry, but I see no reason to fight for slavery now or back then. It's just plain stupid.
And as for forgetting about history, what about the line "History repeats itself?".
Slavery and racism are what fueled WWII. If the European powers had not allowed Germany to make laws against Jews, then the Holocaust may have never occured. But they didn't. They pushed the issue under the rug, and Hitler killed 6million people because of it.