European participation in the African slave trade caused it to become much bigger and more brutal.
Before the Europeans came, the slave trade was relatively small scale and was centered largely around the use of prisoners taken during wars. When the Europeans came, and particularly when they started to take slaves to the New World, the slave trade exploded. From then on, the slave trade became extremely destructive as large numbers of Africans were captured to be sold as slaves. Because they were now being shipped across the entire Atlantic (instead of simply up to Europe) many Africans died in the terrible conditions of the "Middle Passage."
The coming of the Europeans, then, had a tremendous impact on the scope and brutality of the slave trade.