That sounds a lot like relativism to me. You need to investigate before deciding whether slavery was wrong or not?
The Old Testament has 10 commandments, the New Testament has one. Those eleven commandments are absolutes.
Anything outside of the eleven commandments is opinion or moral judgement. Call it relativism, call it wishy-washy, but that is the basis upon which I conduct my affairs.
Is slavery addressed by any of God's commandments? No. Does that mean I approve of slavery? No.