Slave trade is as ancient as humankind (though the word 'kind' doesn't seem to be appropriate here.)
The African slave trade was made possible by the collaboration between white traders and African 'Elites' - those who had the power to capture and enslave others.
The 'White' population in Europe and America regarded the 'Black' slaves as inferior beings, hardly humans, and as such considered their transportation in inhuman conditions to be quite acceptable though lamentable, in the sense that so many (15%) perished en-route.
The African traders too, exhibited an unfathomable contempt towards their brethren.
(If you think that slaves are no longer traded today, google it, and think again.)