Those you mention, who defended slavery, were not doing so because of the Bible but in spite of it.
What in the Bible forbids the owning of slaves?It's acknowledged as a practice in both the New and Old Testaments. And nothing I'm aware of forbids the practice per se. You might argue that the Bible would enjoin Masters to treat their slaves well. But what forbids them from owning slaves to begin with?
See the Pauline Epistle Philemon, in which an escaped slave is encouraged to return to his owner and the owner is entreated to be forgiving. Nothing there condemns slavery. If anything, Christians were encouraged to obey the law of the time even when the law countenanced slavery.
If anything, you can say that Northern Christian Abolitionists who opposed slavery did so not because of the Bible but inspite of it.
Lastly, I'll add that the Abolitionist movement also included many Unitarians (There is but One God and Jesus is NOT Divine) and Universalists (All are saved and no one is damned).