I think a lot of it is differing views on the different types of slavery between someone from the past and a more 20th century view. Back then, slavery was seen as a mercy to prisoners of war. It wasn't really limited to a certain ethnic group. It was prisoners of war and debtors that were stuck there. And depending on the country, many were treated well or poorly. Some had legal protections while others did not.
Mind you, I'm not excusing slavery. It's a deplorable institution that destroys the culture and subjugates the humanity of man. But it is important to remember this perspective.
Of course, even back then, you had abolitionists and free thinkers that blasted the practice as inhumane. And I think that would be perfect for a cleric of Sarenrae that feels deeply that the church has strayed too far from the path of good and redemption. And since words haven't helped, the cleric feels it is time for action. They may still be merciful to plantation owners and nobles with slaves. But ultimately, the cleric goes out and fights for abolitionist legislature by day, and frees slaves from their owners by night.
Maybe she gets a vision from Sarenrae to do this. Many ask why Sarenrae wouldn't just change things herself, and really, she is using your cleric and other like minded individuals to enact change. I say roll with it. It's a great story and a great adventure that I'd like to run one day.