Rodrigo Borgia, a.k.a. the Spaniard. A dark stain on human history. Rodrigo left a trail of blood a mile wide on his quest to unify Italy under the Templar banner. Anyone who opposed him ended up in little pieces inside a sack, or, if he was in a good mood, poisoned.
Once he was crowned pope, Rodrigo, or I should say, Alexander VI, used his influence to wage war with any city that held out against the Templars. And then there are the rumored x-rated atrocities: hundreds of courtesans brought to the Vatican by the cartload and the Pope's close "friendship" with his illegitimate daughter, Lucrezia.
Oh yeah, and did I mention the killings never stopped? Throughout all his public debauchery, Rodrigo was quietly murdering his enemies behind the scenes, consolidating Templar power for the moment when they would seize control.