In the Old Testament those who chose to remain had an awl go through their ear.
As you've mentioned, in some cases slavery can be a good thing. The wise master will protect his investment, care for it, and treat it with dignity.
Emancipation was poorly handled. As a tactical war move deemed essential at the time, it probably happened in the worst way possible, and society is still paying the consequences today.
It is one human being Owned by another