So no one should ever have pointed out that slavery was a bad thing because people's financial interests might be hurt? Or because they might buy more slaves, just to spite the abolitionists? By that logic, we shouldn't say that heroin peddling is bad, because that's a threat to someone's financial interest and it might make them sell more heroin.
You make less and less sense as you go.
While this can be regarded as true after a fashion…
Your grudging admission that slavery could maybe kinda possibly sorta be considered morally wrong is noted.
If you are asking me for advice as to how they should have behaved, I would suggest that the "Holier than thou" method was the first thing they needed to Jettison. It took the Northern states decades to abolish slavery, and they were far less heavily dependent upon it.
Today we have these same Massachusetts Puritans preaching at us about "Gay Marriage" and it was only 20 years ago that Bill and Hillary Clinton were against it. "Everyone conform! Right Now!"
By that logic, we shouldn't say that heroin peddling is bad, because that's a threat to someone's financial interest and it might make them sell more heroin.
Were Heroin a legal product, you might even have a point, as it is, you have another silly strawman fallacy.
You make less and less sense as you go.
Viewing through the distorted mirror of your perception, I can see how you might think that.
Your grudging admission that slavery could maybe kinda possibly sorta be considered morally wrong is noted.
And you have a pervasive need to resort to ad hominem. I am pointing out that many regard the issue as subjective. In my opinion, slavery is absolutely wrong. But that was not the Universal opinion of that era. (and is not the universal opinion of today, either.) You keep wanting to engage in this anachronistic moralizing while ignoring the zeitgeist.