I guess, my response to your post is, I don't agree with the conclusions you make on the events that you cite. In point -- are those examples evidence of X questioning slavery as something that's right or wrong, or X being disgruntled with the the fact that Europeans and Native traders were reaping the benefits of the slave market at their expense?
What I mean is this; you cite Afonso, but far as the facts go, he seems more upset about the self-interest of his citizens overriding Kongo Law rather then the moral inclinations of slavery it's self. There is no evidence I could find that shows he would even care about the slave markets if it just followed Kongo Law. Plus, there is also this,
"Slavery had existed in Kongo long before the arrival of the Portuguese, and Afonso's early letters show the evidence of slave markets."(Atmore, Anthony and Oliver (2001). Medieval Africa, 1250–1800. p. 171.) . When slave markets didn't come at the cost of a kingdom, it seems that no problem was to be found on the issue. This is a case of being upset about exploitation from a specific market imo.
Similar conclusions could be drawn with The Kingdom of Benin too. Yes, you are correct, they did ban the selling of their own ppl(Males only) for a time, but not for the reasons you think.
(Trade and Empire in the Atlantic 1400-1600, pg 45, the time frame is also late 1400s)
What happened in this situation was simply a market dispute, or at the very least the Benin ppl feeling it wasn't in their best interest as a group to sell off their warrior males, especially when you consider Oba Ozolua brought prosperity to his ppl by expanding their military might. I don't see anything to conclude that they thought it was morally wrong if a person was owned.
In conclusion, I'm sure there has been individuals or groups that didn't support the concept of owning another human way before the British or Christians, so in that sense I was wrong. But as a cultural value, I still stand behind that. It's very easy to test a belief as true by words said, but it's a whole other task to test it true by actions. In the case of the Nri Kingdom, you can check the results to see how the kingdom declined compared to it's neighbors when self-interest started knocking at their door. There you will find what values the ppl of the land really stood for which is a much better test imo.
All and all though, regardless of all that, anti slavery ideology that emerged in western civilization from slavery is still important lesson that isn't getting enough attention in these topics, and that was my main point.