I can only speak of the American slave trade, and not anything that happened before 1492, but as far as the Transatlantic slave trade goes, slavery was never seen universally as a positive good.

Bartolomé de las Casas was a contemporary of Christopher Columbus's and participated in early expeditions to America. He criticized the practice of slavery vehemently, and wrote a book called A Brief Account of the Destruction of the Indies that was published in 1542. The light he shed on what was happening in the Americas led to the banning of slavery by Spain, at least for the time being.

Jumping forward and north to British North America, in the first decades of colonization of the future U.S., there wasn't yet full slavery. There was indentured servitude, which still guaranteed some rights to those servants (not splitting up families, you could take your master to court, things like that). In the 1640s and 50s, outright slavery began to be instituted and it was more or less immediately criticized.

Quaker leader George Fox wrote in support of the slaves' struggle as early as 1657.

In 1673, Puritan leader Richard Baxter published A Christian directory, or, A summ of practical theologie and cases of conscience which speaks against the evils of slavery.

In 1684, Thomas Tryon published "The Negro's Complaint of Their Hard Servitude, and the Cruelties Practised upon Them", another anti-slavery text.

In 1688, the people of Germantown, Pennsylvania, signed a document now called the "1688 Germantown Quaker Petition Against Slavery" admonishing the practice of slavery and calling for its end.

From that same year, there is also "Oroonoko or The Royal Slave", a work of fiction published in London that often ridicules the practice.

It took many decades for slavery to become normalized in the American South, and it never became normalized in the North. New York was the only state north of Maryland where it ever took off, but never to the same extent as in the slave states. Future slavery was banned in New York pretty much the moment that the American Revolution ended. But by then, it had become a widely accepted practice in Maryland, Virginia, and all points south, which led to it being protected in those states when the U.S. Constitution was written.