When I think of Sora falling into the Darkness, I think of it more as Sora facing his own inner-Darkness and trying to defeat it. I don't think he'd go dark or evil and Riku would have to stop him, but rather Sora would spend time trying to overcome his emotions.
Somebody said that Sora has gotten past his anger and sadness pretty well, but I have to disagree. It was more like he pushed it aside to stay positive. He hasn't faced any of his anger head-on, and his sadness has been partially dealt with because he found his friends, who were the source of his sadness. Even so, there would still be lingering bits of that anger and sadness. It just doesn't go away like nothing. It sits and festers until it all comes out.
Maybe this is me theorizing, but if the theme of the game is trust, talks about "when light and darkness stand beside each other, light falls to darkness", Nomura's quote of "a Sora that falls, a Riku that flies", then I think Sora will probably have to learn how to trust in himself, and trust that Riku won't leave again. A big event like that had to affect Sora in some way, and what better way for that to be shown than facing his own inner Darkness--his doubts and fears and anger and hatred.