O_O
Ya know I just thought of something really weird
Sticking to the LOTR metaphor, what if it's the Keyblade that makes Sora darker. That part of KH where it says something like "One legend says he saved the worlds; another says he destroyed the worlds" maybe that's saying that the Keyblade itself is dark and affects the user over time. Everyone believes the Keyblade is the key to salvation, but maybe it isn't.
Like the ring, I think it may be been dormant during KH and Com, which is why the dark-hearted weren't allowed to weild it. (Or maybe it's the case of opposites attracting and likes repelling) But during KH2 it will start to affect Sora's actions.
I'm not dismissing either the stab theory or the approaching light theory, I'm ust saying maybe this is another one of the main causes.
(Brain fart over.)
It's probably stupid, but I just wanna see what you guys think.