So I see that Ralph Fiennes has been confirmed to play Voldemort in GoF.
Rowling, Rowling, Rowling. See, you can either cast Jason Isaac, Alan Rickman, and Joseph Fiennes as Slytherins and/or the Face of Evil, or you can bitch about not understanding why people fangirl your evil characters. You cannot do both. If you had cast said evil characters with Danny DeVito, Rush Limbaugh, and Slash from Guns 'N' Roses, you would not be having this difficulty. Next you will be casting Harrison Ford as Barty Crouch Sr. and Josh Hartnett as the Nott kid, and the worldwide conversion to Slytherin House will be complete.