Was the slave trade as bad as people said it was?

Historians gain historical knowledge by reading primary sources, so in short, the only way they can learn about the slave trade is by reading what people said about it. So for the most part, most of what we know about it is reading about how bad people said it was. For millions of West African people, the slave trade involved being taken from their homes and their families, clapped in chains, taken to a slave fortress, or holding prison, and being held until they were purchased by a merchant. At that point, they would likely begin what has become known as the infamous "middle passage." They would be crammed into the lower decks of a slave ship, where they were "stored" for the voyage across the Atlantic. By some estimates, almost 20% of enslaved people did not survive this harrowing experience. These people had no idea where they were going--some believed they were going to a land of cannibals where they would be eaten--and once they arrived at their destination, they were auctioned off to begin work on some plantation or another. In short, historians are basically unanimous in their agreement that the slave trade was a horrific human catastrophe. Even some slave traders themselves, like the Frenchman Jean Barbot, described the slave trade in shocking terms, though he tended to blame its abuses on Africans themselves. He referred to the "barbarous uses of these unfortunate wretches" and remembered that some enslaved people tried to starve themselves to death rather than to submit to slavery:

And tho' I must say I am naturally compassionate, yet have I been necessitated sometimes to cause the teeth of those wretches to be broken because they would not open their mouths, or be prevailed upon by any entreaties to feed themselves; and thus have forced some sustenance into their throats...

Quotes like this powerfully evoke the suffering of human beings caught up in the slave trade, people who would rather die a slow death than experience the horrors that the trade held in store. So the best way to answer this question is to say that people associated with it (even those who profited from it) described it as horrible.