I know why I believe it was, but I would like to compare notes.
Why was the slavery of the pre-Civil War Southern US immoral?
We know that from the advent of the Foundation Era citizens of the Republic were struggling with slavery as a moral issue.
Article I, Section 2 of the Constitution refers to ‘free persons’ and ‘three fifths of all other Persons,’ the infamous ‘three fifths compromise.’ This in conjunction with the Framers never referring to slaves or slavery in the Founding Document is compelling evidence that slavery was not only a complex political issue, but a morally troubling one as well.
Article I, Section 9 of the Constitution prohibits Congress from making illegal the importation of slaves until 1808, clearly an acknowledgement as to the inherent evil and immorality of the slave trade, and by extension, slavery.
The euphemism ‘our peculiar institution’ used throughout the first half of the 19th Century was an admission by slaveholders themselves as to the immorality of the practice, as the institution was in conflict with fundamental tenets of individual liberty.