Ack, this thing has serious potential. I love the concept - turning Sora into the bad guy - but you never really implied just how the Light corrupted him. Some lines also seemed overdramatic, while others weren't dramatic enough. For example, the first few lines weren't vivid enough to hold a casual reader's attention, while the last lines came off as really, really cheesy. With most of the focus on battle and dialogue, the story's emotion fell flat, so Riku's tears seemed overdramatic.
Not that I dislike the concept, or the entire story. It's original, I just feel like you could have done...MORE with it.