The darkness isn't simply his that corrupts "everyone in the KH universe." The darkness is more than someones own power and it extends further than any one person can. To answer the second half of your question, Darkness isn't entirely something that is ultimately evil in respect to how Riku uses it, for instance. Using the powers of darkness he is able to pass through "The Corridors of Darkness" and some are even able to pass through the "Realm of Darkness." The darkness is a little bit more complex than simply being evil. In fact, King Mickey obtained a Keyblade from within the Realm of Darkness called Kingdom Key D. This was a mirror of the original Keyblade Sora used.As for opinions, those are debatable. Aqua regards it as nothing but "hate and rage," while Mickey insists that light and darkness are interdependent and cannot exist without one another.
Here, there are plenty of wikis where you can gather more information & opinions. =] Feel free to ask more questions! ~ NecroGasmic♥ 12:59, December 10, 2013 (UTC)