How did people explain that it was all right to own other people?experiences of african americans from 1619 until after the civil war

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.