There is no way that this is a no-brainer. There have been plenty of people across history who have murdered people for being different based on race, religion or any number of factors really, and 6 million or however many that he killed is not the highest number of dead in a genocide in all of history so there is no reason to assume he is the worst of all-time. He is probably just the one who came closest to realizing his vision.
I'm not advocating Hitler as the most evil of all-time or anything (or even that he isn't), but I'm not sure "body count" is the best stat to use.
Evil is a state of mind, not a summation of your actual evil accomplishments.
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Thank you, Eddie Guerrero.
http://www.wwe.com/f/styles/photo_large/public/photo/image/2012/04/33_Moscow_04112012jg_02128.jpg