Whatever the truth of that matter is, Percy shows bad judgement generally - his idolization of Crouch in GoF, his pompous letter and support for Umbridge in OotP - and his story arc isn't looking healthy at the moment. I think he probably will turn evil and, if he does, F&G knew it before everybody else.

Percy shows bad judgement in refusing to critize his superiors -- but he is the most law-abiding of the Weasleys.

How do you get 'will probably turn evil' out of that?