When European traders first interacted with West Africans it was as equal trading partners. Metal goods, weapons, utensils, fabrics, etc were brought from Europe to trade for ivory & gold. At that time most of the currently recognised Nation states of Europe had already formed but in West Africa the land comprised of a number of separate (and often warring) Kingdoms with no discrete borders. The capture of opposing factions led to such captives becoming slaves and indeed the acquisition of slaves become one of the main reasons for warfare. As an example the Ashanti Kingdom, based in what is now Ghana, became an important political & trading power –initially trading gold with the Europeans but then subsequently trading slaves. Thus the Gold Coast gave way to the Slave Coast and slaves became the most profitable commodity for sale in West Africa. It is fair to say that it was not Europeans who first instigated this slave trade however they were responsible for turning it into a mass industry. It also has to be said that the African form of slavery was different in that slaves within West Africa were, more often than not, eventually freed of their bondage whilst their own children were not considered to be slaves.
As the New World colonies of the Americas expanded they began to produce commodities valued by Europeans such as cotton & tobacco. The expanding economy demanded an ever-increasing labour pool which could no longer be satisfied by indentured servitude (a system of bonded labour that typically lasted for several years for white and black alike, and was a means of using labor to pay the costs of transporting people to the colonies). Indentured labourers ultimately earned their own freedom which created manpower problems for plantation-owners. Over time a series of greed-fuelled laws made it legal for slaves to be held as such for their full lives but only if they came from nations that were not Christian. Subsequent law dictated that the children of slaves would themselves be slaves. The result of these laws was that almost all slaves were effectively of Black-African origin and such slaves rapidly came to underpin the whole economy of most of the New World as well as creating the wealth (directly or indirectly) of many European cities such as Glasgow, Liverpool, Lisbon, Amsterdam, etc.
The creating of an almost wholly black slave class inevitably led to a belief of racial superiority amongst most Europeans of the time. The Christian Church was, at that time the moral arbiter of European society but moral objection from the Church to this racist view was firmly suppressed by ensuring that “slave races” came from non-Christian backgrounds. Firm racist views arose amongst those in positions of power both in Europe & the New World colonies which led to those of African descent being treated as a commodity rather than as people. Once this social view had been reached it allowed for great cruelty to slaves without the raising of significant moral objection. Accordingly huge numbers of West Africans were captured/purchased from their homelands to be shipped –packed like sardines, in the holds of transatlantic trading vessels. Slavery became a multi-million pound/dollar industry and people were treated like simple goods to be bought/sold or disposed of at their owners’ whim!