it's because we are Christian we see it as evil. Prior to Christianity, it seemed just normal. Watch the serial "Rome" and you'll see how normal it was, even morally ok.
Christianity changed all of that. Now we can't imagine slavery.
It’s evil for all the reasons enumerated in the Declaration of Independence, for starters.
The Federalist Papers go into the argument over slavery, and its so clear that even the Declaration is a compromised document.
The God-given rights: Originally, it was going to be Life, Liberty, and Property. Problem with the latter is people could be property. Makes this statement on what is God-given so ironic that its ludicrous on its face.
It was changed to ‘Pursuit of happiness’. Still ridiculous in the face of one class of people owning another class of people as property, but it was at least defensible.
All people should at least start out as free people. God gives us free will, and he made us.
And it makes no sense to evaluate this outside of a religious context since the country was founded invoking rights derived from God.
The ‘Everybody’s doing it’ defense for slavery is specious.
However, there is the context that if a culture possessed a clear military advantage over another culture, they were almost morally obligated to exploit it. It was your responsibility as the defender of that culture to do so, with the expectation that if that innovation were in a neighboring culture’s possession, they’d be so obligated.
From that perspective, you could view slavery as moral, preferable to erasing the thread through some Carthaginian-style genocide, where every living human is killed.