I'm pretty sure the writers didn't intend for Leo to turn out to be corrupt. I'm just retconning in the cracks and crevices of canon, just as they did when they shoehorned Beaver into the rape scenario. And Leo was always drawn in sharp contrast to Logan as a possible love interest for Veronica; if you saw him as a sweet boy, you were just paying attention. I feel it's ok to do such a huge about-face with this character because of the general noirish precepts that all characters are flawed and are presented with impossible moral dilemmas. Not evil. Flawed, and facing impossible situations, just as we all are. Reconsider the rape for instance: as they point out on MI.net, it's the ultimate noir solution (in season one): everyone plays a part, no one is blameless, not even Veronica (she drank a drink that she didn't watch being poured, which surely her dad warned her about), not even those who just witnessed Veronica's condition that night, such as Meg, and bear some guilt for not stepping in to stop it.