Most Horrifying Aspect

Holbo's first reading isn't plausible, as he admits, but the only legitimate reading I can come up with is truly horrifying. Medved:

Historians agree that hundreds of thousands, and probably millions of slaves perished over the course of 300 years during the rigors of the “Middle Passage” across the Atlantic Ocean. Estimates remain inevitably imprecise, but range as high as one third of the slave “cargo” who perished from disease or overcrowding during transport from Africa. Perhaps the most horrifying aspect of these voyages involves the fact that no slave traders wanted to see this level of deadly suffering: they benefited only from delivering (and selling) live slaves, not from tossing corpses into the ocean.