Yes, God did free Israelites from their slavery in Egypt, but that did not mean that God wanted to destroy the institution of slavery.
Both the Old Testament and the New Testament are clear that God supports the institution of slavery. Slavery is not sin in the New Testament. In fact, St. Paul told a run-away slave to go back to his slave master.
Just because you do not like slavery does not mean that God opposes slavery as an institution. God has His own ideas of what is right and wrong, and we can read about it in the Holy Bible.
Slavery is not sin. That is emphasized in the New Testament and the Old Testament.