The Rosenbergs???
How many people died at their hands? Or by events stemming from their actions? (That does appear to be the consensus criterion: how many deaths can be laid at the feet of each.)
I too, would omit Hirohito, as merely a puppet of the warlords. I would move Mengele, Eichmann, Himmler and Milosevic much farther toward the top.
Conspicuously missing: Augusto Pinochet, Ferdinand Marcos, Francisco Franco... and two names I doubt any of the bloggers polled would be familiar with - in fact, I didn't know them either until I began researching for this reply - Daniel Francois Malan and Hendrik Verwoerd, the chief architects of South African apartheid.