There is a slave ship run by Africans, holding African children, going up and down the coast of Africa at this present moment. In fact, slavery started in Africa, and the slave trade in the US could not have succeded without the help of Africans capturing and selling off their own kin to slave hunters.
Slavery is wrong no matter who enslaves others.
Pointing out that Africans were involved in aiding Europeans in the American slave trade does not in any way alievate or disqualify the guilt that Europeans and Americans held in that trade. There wouldn't have been a slave trade for the African's to assist had the Europeans not been there. Why, because there is no connection. Sometimes people are people and they can't work out their differences. In the Balkans, it's the same story, but we can't blame the Turks or the Soviets for having conquered the region in their turn.