Slavery was justified in a number of ways. Among them were:
It was allowed by the Bible and by the Constitution. To this way of thinking, any institution that had not been condemned by the Constitution or even in the New Testament could not be wrong.
It was good for blacks. Many whites argued that blacks could not really take care of themselves. They were supposed to be so inferior to whites that it would be hard for them to live in America as free people.
It was better than freedom. Southerners argued that slavery was more just and humane than wage labor. They said that slaves were cared for no matter if they were too sick or too old to work. They contrasted this with wage laborers in the North who were on their own if they were not able to work.