Nonsensical. Slavery Was Very Bad, but the idea that we can definitively identify the worst thing in history - and that it was the 10-20% of the historical human slave trade led specifically by whites - is just damned absurd. Some other contenders: The Mongol Conquests The Fall of Rome WW2/the Holocaust The Black Death New World population collapse Communism The Steppe and Bantu Migrations The Arab conquests/slave trade Etc.