Nobody so far has pointed out the obvious:
The first people to believe that slavery was evil and wrong were probably discontented slaves (as opposed to contented slaves).
I would guess that nobody was as likely to notice and disapprove of his boss's unjust treatment of him as a slave would be.
In ancient Rome the First Servile War was in Sicily from 135 to 132 BC and involved about 70,000 to 200,000 rebel slaves. The Second Servile War in Sicily lasted from 104 to 100 BC. The Third Servile War was on the mainland of Italy from 73 to 71 BC and involved 120,000 slaves led by Spartacus. These rebel slaves were mostly interested in gaining their own freedom and the freedom of their friends and families. But it is quite possible they sometimes expressed the opinion that all slavery was wrong, and that other people heard of that opinion.
The Zanji Rebellion in southern Iraq from 869 to 883 was one of the biggest and bloodiest revolts against the the Abbasid Caliphate and the rebels included black slaves and free men. It is certainly possible that some of the slaves involved expressed the opinion that all slavery was wrong.
And there have been many other revolts of slaves who mostly sought their own personal freedom, but may have sometimes said that all slavery was wrong.
The Institutes of Emperor Justinian (ruled 527 to 565) say:
. . the law of nations is common to the whole human race; for nations have settled certain things for themselves as occasion and the necessities of human life required. For instance, wars arose, and then followed captivity and slavery, which are contrary to the law of nature; for by the law of nature all men from the beginning were born free.
Slavery is an institution of the law of nations, against nature, subjecting one man to the dominion of another.
And these can be interpreted as anti slavery opinions. Certainly many 19th century southern Americans expressed opinions far more favorable to slavery.
An early American protest against slavery was the Germantown (Pennsylvania) petition in 1688 by four German Quakers and Mennonites.
Slavery has been abolished by many Chinese governments for various reasons, the first time by the Qin Dynasty of 221 to 205 BC.
The Sachsenspiegel lawbook of about 1220 condemned slavery as a violation of God's likeness in man.
King Louis X of France decreed in 1315 that "France means freedom" and any slave entering France was to be freed.
And so on. There are many possible anti-slavery expressions throughout history, making a definite history of opposition to slavery difficult to wwrite.