By the number of death? By the nature of those deaths? By the reason for those deaths?
Hitler was responsible for the extermination of "only" 6 million Jews. Is he a piker monster-wise or was there something more evil about him than there was about Ghengis Khan?
I GOOGLED "who is the most evil person who ever lived" and a woman's name came up who murdered babies.
Ask "who were the most evil people who ever lived" and the list is very long.
And getting longer.
We are going in the wrong direction and our speed is accelerating. To what end? Annihilation?