Furthering and clarifying the question: how many of the enslaved people who were brought to the Americas were former free men and how many were already working as slaves in Africa? Were they from tribes or from organized kingdoms?
How did the Portuguese and other European people see slavery when they began contacting Subsaharian cultures, how did they assimilate the institution, and did they make it worse than it was in Africa?
Slavery in Africa wasn't supplemented or based on racism. Also they tended to preserve slaves' life and wanted to "use" them or ask for ransoms rather than spill blood, and slaves could buy their freedom or occupy important positions in their societies. In contrast, black slaves were transported across the Atlantic Ocean in harsh and inhuman conditions, resulting in high mortality rates.