Slavery is definitely wrong. However, not all slavery systems were equal. I think you are describing slavery under Britain and the USA and other kindred countries which had virtually no laws protecting the slaves. Portugal and Spain had laws which protected those under slavery. For example, Under these the slave was allowed to work during the weekends in order to save so he could eventually by back his freedom. The separation of close family members was prohibited-as well as arbitrary cruelties.
The website from which the following excerpt was taken describes the differences between the two systems, a difference similar to the difference between night and day.
Excerpt:
The Spanish Crown included the slaves as persons instead of relegating them solely to the status of property at the disposal of their owners... when a slave ship reached the New World, a friar boarded the ship and examined the slaves to see that the requirements had been met. The Church also insisted that the slaves become regular communicants, and it liked to view itself as the champion of their human rights...
The law also defended the right of the slave to purchase his own freedom. He had the right to own property and could accumulate funds with which he might eventually achieve his dream. He also had the right to demand that his master or the courts set a fixed price for his purchase which he could then pay over a period of years. Sundays and holidays were for the slave to use as he saw fit, and, in some cases, he was also guaranteed a couple of hours every day for his own use. During this time he could sell his services and save the proceeds. The law also stated that parents of ten or more children were to be set free. Finally, slaves could be freed by the courts as the result of mistreatment by their masters.
http://www.rit.edu/~nrcgsh/bx/bx03b.html