How does the slave trade highlight the dark side of Englightenment?

The darker side of the Enlightenment time period can be seen in how its ideas were used to justify enslavement practices.  
One idea that showed the darker side to the Enlightenment was its belief in the authenticity of universal truth.  The Enlightenment period saw truth as a universal concept.  It was applicable to anyone and everyone.  Enlightenment thinkers advanced certainty and absolutism as universal notions of the good life.   Once someone understood what they perceived to be true, they had an obligation to spread it everywhere because it was "enlightening."
For some, this became a justification behind slavery.   Some individuals were convinced that they were freeing indigenous people from abhorrent conditions.  People justified it as a way to remove indigenous people from "unenlightened" reality.  For example, in "A Defense of Slavery," published in London Magazine in 1740, the argument was laid out that "The Inhabitants of Guinea are indeed in a most deplorable State of Slavery, under the arbitrary Powers of their Princes both as to Life and Property."  Enlightenment theories were used in this piece to suggest that indigenous people lived in "unenlightened" conditions.  Those who justified slavery could very well look at the indigenous experience as "contrary to Nature and Reason."  Accordingly Enlightenment ideals were used as a way to justify slavery:  

But are not all arbitrary Governments, as well in Europe, as Africa, equally repugnant to that great Law of Nature? And yet it is not in our Power to cure the universal Evil, and set all the Kingdoms of the Earth free from the Domination of Tyrants, whose long Possession, supported by standing Armies, and flagitious Ministers, renders the Thraldom without Remedy, while the People under it are by Custom satisfied with, or at least quiet under Bondage. 

This use of Enlightenment ideas reminds us of how theories can be manipulated to consolidate power.
The thinkers of the Enlightenment period did not believe that their theories should justify slavery.  In fact, with its emphasis on freedom and autonomy, Enlightenment ideals repudiate it.  However, some of the ideas in the Enlightenment were used as ways to theoretically support the practice of enslaving others.  Individuals who used it in this manner believed that Western thought was inherently superior to anything else, deeming other social and belief systems as rudimentary and not advanced.  With this emphasis on certainty and absolutism, the Enlightenment's dark side is evident in how it was used to justify the practice of slavery.
https://plato.stanford.edu/entries/enlightenment/