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.