From the wording of the second part of this question, I infer that you are asking about the Atlantic slave trade. I will base my answer on this assumption.
One way to characterize the origins of the Atlantic slave trade is to say that it was caused by two main factors. One factor was the advent of European exploration. The other factor was the preexisting Muslim slave trade that centered on sub-Saharan Africa.
Slaveholding had long been a tradition within sub-Saharan Africa itself. Slaves were obtained through such means as war and were used as a major indicator of wealth and status. Therefore, there was considerable trading of slaves within Africa. This led eventually to the development of a more long-distance slave trade in which Muslims merchants bought slaves in Africa and sold them elsewhere. One book from which I have taught (Traditions & Encounters, by Bentley and Ziegler, Brief Second Edition, p. 288) says that Muslim traders may have taken as many as 10 million slaves from Africa between 750 and 1500 CE.
In the mid-1400s, Europeans started to explore down the west coast of Africa. Eventually, they got as far as the regions from which the most slaves came. Once there, they tapped into the existing networks of slave trading. This was the beginning of the Atlantic slave trade.
In my view, it is very hard to distinguish between the roles of Africans and Europeans in the Atlantic slave trade. It is hard to deny that both groups were extremely culpable for the enslavement of millions of Africans. The Europeans were the source of the demand for slaves and they were the source of transport for those slaves. However, it was Africans who supplied the slaves. The Africans raided other Africans to take slaves to enrich themselves. The Europeans transported and sold slaves to enrich themselves. There is no moral difference, in my view, between capturing and selling the slaves on the one hand, and buying, transporting, and reselling them on the other. Therefore, I would say that the role of Africans (the ones who captured and sold slaves) was morally equivalent to the role of Europeans (the ones who transported and resold slaves and the ones who profited from their labor) in the Atlantic slave trade.