The Southern preachers who defended slavery misused the Bible. They themselves were slaves to the culture in the sense that they could not see or did not want to see the problems with slavery.

The Bible does not teach explicitly against slavery because the Bible is not about overthrowing social structures that are evil. It's about redeeming people to new life. Many preachers did advocate abolition of slavery and used the Bible for that, too. I think that one can find principles against slavery in the Bible. The main teaching in the Bible was for masters to be good and kind to the slaves and the slaves to obey masters. Slavery then was a different form from the South - and many were willing "slaves" in order to pay off debt.