It's difficult to point to one individual who was the first to oppose slavery. Those who claim such a thing may just be those who are in a position to record it, or have others record it. No doubt there was a landowner who decided that he would not use slaves, that this practice was repellent, and told his fellow landowners this. Who knows?
What is in the history books is down to the scriber, and this is that in 1657 the English founder of the Quakers, who believed in equality, pronounced the practice evil. But Vermont, give it credit, decried slavery in 1777 way ahead of other states and was the first to abolish it.