I (like a few others) think that Riku and Mickey are double agents, and not truly Organization members. And Onix, your theory doesn't make much sense. For one thing, Mickey is the King of light and Riku is an entity of Twilight. Secondly, before they'd left Castle Oblivion, Mickey had saved Riku from Ansem's darkness. Furthermore, Riku and Mickey took the path to Dawn when they met up with DiZ, not the road to darkness. So no, they can't have turned evil.