1. The worst, in the meaning of killing some one for no other reason than "killing him", no political, economical or whatever gain: in that case I see the Holocaust. That crime was committed for "free", just for the sake of killing, and that's what makes it Unique. All other crimes had a reason: econimical reason (Triangular Trade), territory reason (Crusades, Native Indian Massacres), religious reason (The Crusades, the Inquisition, The Jihad), political reason (Cultural Revolution, Pinochet, Pol Pot, Staline)
2. The one crime that had the major effect on the world today: killing the Native Americain, imagine how the world would look like if the europeans didn't took the place of the natives in the Americas.