But owning slaves is an inherently evil thing to do. So how do you reason that.
Owning slaves was a major part of the economy on every continent, and people at the time (at least up until the abolitionist movement) did not view it as "evil." The idea of person liberty is a relatively modern one, and the majority of people who have ever existed on this planet did not share those values.
So the reasoning is that you're falling guilty to the fallacy of presentism. You're imposing 21st social norms and values on a time period where those are inappropriate.
"but others did it too! why be accountable?"
"time dictates good and evil!"
Yes, it very much does.
I'm certainly hoping we don't have a generation of SJWs in 2090 who think MLK Jr. was "evil" because he opposed gay marriage. It's unreasonable to apply those standards to someone at a time when they were not the norm. ---