I gotta say Holocaust, my reasons:

(1) The Holocaust killed over 6 million people. Slavery killed people yes, but not as many.

(2) The Holocaust performed unbelievably wrong experiments on people.
Example: Injected a pregnant woman with concrete to see how long it would take for it to kill her and the baby, and who would die first.

(3) The Holocaust killed thousands of people and didn't even try. People living underground and shit died as well.

(4) Even now people in Ukraine will kill you if they hear you hid or your family hid Jews.

^^My 2 Cents. px