IIRC, the first slaveholder in the Bible was Abraham. If I'm not right, someone will correct me.
God put his own people into slavery for disobedience and he allowed his people to subdue other nations and take slaves. So, when, if ever, did it become a sin?
A good question, and a difficult one to answer.
I'm going to try and explain this, but am not suggesting you simply accept it. I do not know if you are a Christian, and I don't expect you to believe in Christian concepts, if you are not a Christian.
This is just an explanation.
SO....... In Christian terms, slavery is never considered a sin. It is regulated, and controls, and limited.
In fact, the way the way the Bible refers to Christianity itself, is that we are "slaves to Christ". We are slaves to our Lord and King, Jesus Christ the Messiah.
A proper Christian that truly follows Biblical teaching, understands that he life was bought and paid for, by the Blood of Jesus on the Cross. So we are slaves to the Lord.
Now as for how the Bible regulated slavery between men.... Some slaves were due to debts. Others to pay for crimes. Some were captured in war.
However, slavery was not permanent status, or even a negative many times. You didn't lose your humanity being a slave.
For example, once you worked off your debt, and the debt was paid, you were no longer a slave. If you were a slave for doing a crime, it was usually over whenever you finished your time. If you were a captured slave, you were released on the year of jubilee.
In all cases, slavery was never a fundamental aspect of your life.
Now compare that to what we saw elsewhere in history. Black people for example, were simply thought to be less than human. They slaves because they were black. They were considered lower-beings. Similarly, in India if you were born lower-caste... you were made to do the worst possible jobs, simply because you were a lesser-human in their view.
Nothing of that sort, existed in Biblical term. In fact, many slaves were treated with great respect and honor depending on who they worked for, or what they did. This is so true, that many slaves would voluntarily give up their freedom for life. Deuteronomy chapter 15 talks about this.
So my suggestion to you is that was the great evil, the slavery of Darwinism, where we are superior more evolved people, and those lesser beings are slaves.
The bible goes to great lengths to clearly say that we are all equal under G-d.
1 Corinthians 12:13
For by one Spirit we were all baptized into one body, whether Jews or Greeks, whether slaves or free, and we were all made to drink of one Spirit.
The problem with modern slavery, is that it made one person lesser, than another. That is evil.