Something that we now look at with horror and revulsion enjoyed a great deal of justification. Slavery was justified on many grounds. The abduction of people into bondage could only be continued and expanded with such rationalization.
One justification was on religious grounds. As the Atlantic Slave Trade increased, the perception was that Africans were "heathens." This view of Africans was one that suggested any non- Christian was an impediment to Christianity. Such a view also helped to demonize Africans, making their subjugation easier and more widely accepted. When Prince Henry of Portugal seeks the Pope's blessing for more slave raids and further enslavement and receives it, one sees how religious faith played a role in the justification of slavery.
Another justification of slavery was money. There were large fortunes to be made in the business of slavery. For many Europeans, the commodity of human trafficking was undeniable. If they were not going to engage in this process, someone else was going to do so. Slavery was seen as a way in which large sums of money could be made. Given the fact that Africans themselves had no real way of stopping the growing power of European nations, this helped to further justify slavery. It was seen as a business, as a form of commerce. This helped to further increase the slavery business, justifying its expansion as a matter of commercial enterprise.