The Ashanti were also a pre-literate society. The reason we know so much about them is that they had a long trade relationship with Europeans on the coast, particularly through the slave trade. The idea that slaves were treated better among the Ashanti than other tribes or places is highly questionable, since Ashanti slaves were killed when their master died. Mass human and animal sacrifices were also a part of their culture. The British had such a difficult time conquering the Ashanti because they were such great warriors, due to a lot practice fighting against surrounding tribes to gain territory and captives to sell.

The Ashanti are a fascinating subject, but they weren't magically good somehow, and you do them a diservice by leaving out the parts that make them just like every other empire ruled by a despot.