Hmm... I think I'm going to have to look at the human damage scale first, and base my vote on that.
Main contenders -- 20th century only? OK...
How about: Prime Minister Neville Chamberlain, who not only refused to believe that Hitler was a threat to Europe or the UK, but had people who disagreed accused of treason? And tried to get a young Winston Churchill drummed out of politics?
Or... that fine upstanding Father and Son combination, Papa Doc and Baby Doc Duvalier, who despoiled the land and people of Haiti to the point of utter devastation?
Charles Taylor, of Liberia?
P.W. Botha, president of South Africa, who incarcerated Mandela, and refused to testify to the Truth Commission after the fall of apartheid?
Then there's Pol Pot, whose innovative decorating ideas - involving the picked-clean skulls of his country's citizens - gave us the term 'killing fields'.
Augusto Pinochet, whose Chilean regime, along with the juntas of Uruguay and Argentina, gave us the term 'the disappeared' -
http://en.wikipedia.org/wiki/Forced_disappearance Nicolai Ceaucescu, dictator of Romania, who made life hell on earth for millions.
Mao Tse-Tung, who made life hell on earth for hundreds of millions. Under his Great Leap Forward, in parts of China, starvation was so acute that people resorted to cannibalism.
Like they did in Russia, under the reign of Uncle Josef Stalin.
But I guess my all-time hands-down favorite contender for the title would have to be everybody's one and only, the worst person in the world... so far, although quite a few others come close...
Adolf Hitler.
If I don't limit myself to human beings, though, the eternal prize would have to go to the Liar, the Father of Lies, the Covering Cherub, first of the fallen angels, hater of all that is good. For all these listed above are in every way his children, and spent their lives doing his work.
