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.
"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."
Not only is there reason to think that, it is demonstrated. Slavery had been in existence for millennia; it was a fact of life. The consciousness that it was a moral wrong did not explode in the minds of every human being simultaneously. It took root slowly, and grew slowly.
When a person was born into a slave-owning society, and brought up to believe all the arguments justifying it, you can argue with him and tell him he's wrong, but you can't point the finger at him and say, "You should have known." That's demanding too much.
"And anyway, the whole point about this pamphlet is that...(it) attempts to argue that slavery was not so bad and slaves were happy being slaves."
Ah, I think I begin to see the disconnect here. It's right there in that phrase "not so bad."
The question of whether slavery is morally wrong and the question of how slaves were treated are separate. The one does not depend on the other.
You seem to think that an assertion that a slave owner treated his slaves well is a justification of slavery. That is not the case. Even if a slave owner treated his slaves like his own children, slavery is still a moral evil.
However, and this is a big however, the truth is the truth. Even if we hate like poison that slaves were not treated as badly as we have been led to believe, if that is the truth, we must still face it.
"That has nothing to do with the particular moral viewpoint or motivations of slaveholders in the 1800s."
The question of whether slavery is a moral evil has nothing to do with that. However, the question of the character and moral standing of slaveholders has everything to do with their particular moral viewpoint and motivations.
"It asserts something that is not only unsupportable by fact but reprehensible."
It does not assert, as far as I know, that slavery is not a moral evil. It does assert that slaves were treated better than many people have been led to believe. And, since that is true, it is not reprehensible to assert it.
Southerners have been demonized as the sort of slavering brutes you seem to think them, by assertions that they treated their slaves much worse than they actually treated them.
From your point of view, Southerners (all the millions of them) were perverse, malicious brutes, abusing their slaves for amusement.
From mine, they bear the moral onus of owning slaves, but they only take the rap for such abuse as actually occurred.
It seems that you can't imagine how a person could own slaves and still be a decent person in other regards. Southerners must be demons.
I don't know what else to tell you, except to try and wrap your mind around the concept that a slaveowner could still feel real empathy for his slaves, without it occurring to him that the institution was a moral evil.