No, slavery wouldn't have been morally right then, just as it wasn't morally right when the US was founded, and just as it isn't be morally right today.
That's your idea of what "truth" is? So to you, liberty and human freedom is whatever you or someone else wants it to be at the moment? What will you decide it should be tomorrow?
You believe that you have the moral authority to determine who deserves what and how much and then force that view on everyone else? Sounds like you would have been at home in 400 B.C. with a hut full of slaves.