"But some things are right or wrong no matter what the century and I'd think "human beings should not be owned like cattle" would be one of those basic things."
So it seems to us now, but that was not always the case. There is a partial response to that in note 111.
"Especially in a country whose founding document asserts that all men are created equal."
Among those who didn't think that Negroes were "equal" enough to enter white society was Abraham Lincoln.
"So no one can argue that people in that time period just didn't understand the moral questions."
Actually, you can. You can also argue that, having studied the matter, they came to a different conclusion from us. What you can't argue is that they were motivated by sheer perversity and malice.
"And I'm amazed by the attitude of so many here who usually lambast "moral relativism" when practiced by the left."
Those who oppose you are not engaging in moral relativism; they are simply taking human limitations into account. You can't demand more of people than they were capable of giving.
Why not? I can absolutely argue that. The 1800s were not that long ago, you know, and there's little reason to think human beings had no understanding that owning other human beings was wrong. That they chose to ignore that truth can certainly be seen as evidence of perversity and malice, probably allied with the age-old human condition called "greed" and simply getting away with whatever you're allowed to get away with.
And anyway, the whole point about this pamphlet is that it's not a 19th century account of a slaveholder's point of view, read in context along with slave narratives for historical purposes. It's a modern treatise that attempts to argue that slavery was not so bad and slaves were happy being slaves. That has nothing to do with the particular moral viewpoint or motivations of slaveholders in the 1800s. It asserts something that is not only unsupportable by fact but reprehensible.