The slave trade was appalling. It should be unbelievable that one group of humans could treat another in the sorts of ways in which Black Africans were treated over these centuries. Unfortunately, there is much too strong a pattern running throughout history of power being abused for this to come as a surprise.
However, the BBC's implication that Christianity somehow was more complicit in this than other people seems something of a slur. Aside from the fact that William Wilberforce was an evangelical Christian, this poem by Joseph Samson traces a line of independent Christian opposition to slavery (or in some cases, at least to the worst practices of slavery) that goes back about 150 years from the time of abolition. For example:
In 1758, the People called Quakers, in Pennsylvania, came to a final resolution to deny the rights of membership in their religious Society, to all such of their Members as should persist in detaining their Fellow-Creatures in bondage, after Gospel admonition against the unjust practice. Many strenuous Advocates for the oppressed Negroes appeared about this time among the different Professors of Christianity, whose pious endeavours for their relief were at length blessed with considerable success: but of late the generous ardour for liberty, which characterizes the present age, has spread with unexampled rapidity. Where solitary Individuals lately wept over the suffering Negroes, numerous Societies are now established to befriend the Enslaved, and to protect the Free. They have solemnly represented the horrors of the Slave Trade to the Legislatures of Great Britain, France, and the United States of America; and unless the clamours of Self-interest and mistaken Policy can stifle the groans of Distress, and obliterate the dictates of Humanity, decisive measures will soon be adopted for the abolition of a trade that has deeply stained the annals of the eighteenth century with robbery and murder....John Locke was also a Christian, but is now regarded as a leading light of humanistic tolerance. He, however, despite his enlightened views, was "an investor in the Royal Africa Company, along with most of the English court and the political elite" (Robert Lacey, Great Tales from English History vol 3 p.20). People at the time argued that the Bible endorsed the practice of slavery. People will still say that the Bible must be discarded because this is the case. Actually, it makes no such endorsement, and even from the start of the practice of the slave trade, thinking Christians could see that it was incompatible with biblical Christianity.
Richard Baxter, an eminent dissenting minister of the last century [that is, the 1600's!], some of whose Discourses are now extant. His Directions to Slave-Holders contain a great deal of christian admonition respecting their treatment of the Negroes, and were first published at London in 1673. "They are reasonable creatures as well as you," says he, "and born to as much natural liberty; they have immortal souls, and are equally capable of salvation with yourselves; equally under the government and laws "of God;" exhorting them to consider "how cursed a crime it is to equal Men to beasts."