A few of you guys mentioned Irenicus, and he does deserve it. However I still stand by the belief that Sarevok was more evil. Let me list the ways:
-Had you fosta-pappa Gorion killed
-Started an economic crisis by poisoning the resource (in this case, iron)
-Hired mercenary groups to halt the shipment of said resource
-Took over a mine, kidnapped people and forced them to be slaves
-Brought shapeshifters to kill off and impersonate business rivals in order to drive his company to the top (and you thought insurance companies were bad, lol)
-Set up his own foster-father's death, along with the other owners of his company so that he could run it to gain power within Baldur's Gate
-Murdered everyone in your townhood home, from your mentors to your teachers to your buddies and friends. Then had all of them impersonated by shapeshifters to throw you off.
-Worked to become one of the rulers of Baldur's Gate then tried to kill off the other leaders
-Plotted to start a war with Baldur's Gate and Amn over the economic crisis, all to harvest enough souls to become a god
-Throughout the entire game, he sends pissant little assassins to try to kill you. All because you are his half brother.
Irenicus did some bad voodoo but Sarevok is one bad muthafugga.