Your right. The Bible has never changed and we humans are not perfect. Many, many, many things are misinterpreted about scripture. I'm sure I mangle it at times as well. This is why you have to use your faith and ask God for the wisdom to understand.
If I remember correctly, it was more than the Bible that was used to justify slavery.
Absolutely, and I don't even claim that the Bible was the primary justification. It probably was a very minor one. The point is that God's word was then used to argue the idea that slavery was right. We no longer believe that God's word says that slavery is right. Our understanding of the Word has changed since the time of the Civil War.