There were many ways in which people justified slavery. Some people argued that slavery was a necessary evil, but others argued that it was positively a good thing, even for the slaves.
Some people argued that blacks were not really capable of taking care of themselves and that, therefore, they were better off being enslaved. If they were enslaved, at least they would have someone to take care of them. A variation on this argued that blacks were better off in the US, where they could be Christianized and civilized, than they would have been back in Africa.
Another line of argument said that slaves were treated better than the wage laborers who worked in factories in the North. This justification argued that slaves were cared for when they were too sick to work and when they were too young or too old to work. Slaves, unlike wage laborers, had lifetime security.
There were many other lines of argument, but these were the two most prominent ones.
There have been documentaries on the History Channel as well as discussions in books concerning who or what shaped the opinion that it was all right to own African Americans as slaves. I remember a narration on television from Mr. Wesley Snipes, who is an African American, and well versed on American Black History. Mr. Snipes stated that the Arabs, who were substantial slave traders, and White slavers started a myth that Black Slaves were not humans with souls and did not go to heaven. By degrading the status of Black Slaves, they were able to convince people who had a need for slaves and could purchase them, that there could be no conflicts in regards to their Christian values if they owned slaves.
Slave owners also used the excuse that Black Slaves did not have the mental and cognitive skills to care for themselves and needed owners or slave masters to care for them.
However, a few writers in the North wrote books that detailed the cruelty of slavery and addressed the humanity and feelings of Black Slaves. These writers further outlined how slavery in the South was against Christain morals and principles. The circulation of books against slavery stirred the conscience of people in the North, as most of them did not own slaves. Their feelings and opinions against slavery started the Civil War in America to free the slaves.