Well, none of us were alive back then, were we? So we can only speculate what they were/weren't thinking.

It could have been a mixed bag. For many of them, the presence of slaves in their daily lives could have normalized slavery (in their minds) to the point where they legitimately saw it as "no big deal."

But I have a hard time believing that a critical mass of people during any time period were categorically ignorant of the inhumane exploitation in which they were engaging. Certainly, not every single slave always behaved like a docile puppy.