I think people may be thinking way to much about the "erase me" part of his conversation. I really think he just starts getting incredibly depressed for some reason, because he is losing his powers over the keyblade, and feels like a burden. It could just be that he gets fed up with all the fighting or it could possibly be like some have already metioned, and Ventus thinks that by erasing himself it would all be for the greater good. These are all potentially the reason.
Well then there was no point showing us that part. also why only show aqua's shadow? i'm pretty sure something dark happened there. and your view on erase me is a bit too simple from the times they sued the word erase (COM)