Most evil character

John Amos and then Malcolm. (John Amos because he influenced both Malcolm and Oliva and set the chain of events in motion that led to the poor children being locked in the attic. I wouldn't be half-surprised if he conspired in some way with Corrine to have her poison the children, perhaps duping her into believing that it would make them all sick enough to get out while lying about the dosage... that is, if you believe that's all she was doing.)