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?
Your question invites a distinction between African slavery in the U.S. and slavery in general, which existed in most parts of world for millennia. It should also be noted that this type of slavery was a British invention which was inherited by the United States after its independence. The continuation of British slavery in the Caribbean was, if anything, even less humane than conditions in the Southern States.
That being said, U.S. slavery was especially pernicious in that slaves were often treated worse than animals: They could be worked to death and sexually molested without any moral implications whatsoever. This obvious contradiction was deliberately ignored by many people who otherwise claimed to be "good Christians."
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