There were several factors that made the Atlantic Slave Trade one of the worst human tragedies in world history. Slave traders had no regard for human life.
They would pack their ships so tightly that slaves were forced to make the trans-Atlantic passage chained into triple-decker bunks. Diseases, human waste, live and other pestilences would spread throughout the cabin and result in 50% of the human cargo dying in transit. Captains refused to feed those slaves who looked too weak to survive or who got sick. Most of the time they would simply through them overboard. Those who did survive were held in pens until they were healthy enough to be sold at auction.
Based on what records we have, historians estimate that 15 million slaves were taken from their homes in Africa during the slave trade. Of those, only around 8 million actually surived to arrive in the New World.