Not Black & WhiteAqua had always been taught by Master Eraqus that darkness was bad and needed to be destroyed. Nothing good could come from darkness and it needed to be kept in check using the light. The reason Terra did not pass the Mark of Mastery exam was because he had a moment of "weakness" in which the darkness in his heart took control. A Keyblade Master would not have darkness in their heart, according to Eraqus. It's no surprise then that Aqua would have such naive beliefs that darkness is bad and light is good. She views the issue in such a black and white fashion that it is almost hard to believe considering how open-minded and willing to learn she is.
When Aqua meets Cinderella's Fairy Godmother, she is dumbfounded when she is advised not to fight the darkness with light. She responds to Aqua's questioning by saying,
"Strong rays of sun create dark shadows. Sadly, Lady Tremaine and both her daughters are jealous of Cinderella's charm and beauty, qualities that appear to you as 'light.' Jealousy is darkness. Light and dark go hand in hand. You can't have one without the other."Despite being taught for so long that light is supposed to destroy the darkness, Aqua strongly considers what she has been told. Even so, she still does not truly understand. One can't exist without the other, but what did that mean for her? Wasn't she supposed to destroy the darkness? Looking down at her Keyblade, she wonders what she is supposed to do in order to help if she can't fight using the light. The Fairy Godmother advises that she helps one of Cinderella's friends to ensure that Cinderella's light does not go out instead of destroying the darkness. After all is said and done, Aqua comments that it is strange to change her way of thinking. Eraqus had always taught her to destroy the darkness, but how was she supposed to do that without using the light? The Fairy Godmother smiles and tells her to experience more things and eventually she will find her answer.
Aqua visits more worlds but still bases her judgment and actions off the assumption that darkness is bad because of all the destruction and havoc that she saw right in front of her caused by it. She boldly tells Maleficent that true love will always defeat the darkness without hesitation, for example. However, it is not long before her conceptions are challenged once again. After meeting Kairi in Radiant Garden, she listens intently to Kairi's grandmother tell the following story:
"Long ago, people lived in peace, bathed in the warmth of light. Everyone loved the light. Then people began to fight over it. They wanted to keep it for themselves. And darkness was born in their hearts. The darkness spread, swallowing the light and many people's hearts. It covered everything, and the world disappeared. But small fragments of light survived, in the hearts of children. With these fragments of light, children rebuilt the lost world. It's the world we live in now. But the true light sleeps, deep within the darkness. That's why the worlds are still scattered, divided from each other. But someday, a door to the innermost darkness will open, and true light will return. So, listen, child. Even if the deepest darkness, there will always be a light to guide you. Believe in the light, and the darkness will never defeat you. Your heart will shine with its power and push the darkness away."Aqua hears once again that darkness and light both exist together. Echoes of "strong rays of sun create dark shadows" can be heard throughout the story. However the story talks even more of the relationship of darkness and light. Even in the deepest darkness, light still exists. Even in the brightest light, darkness is lurking around the corner. One cannot completely conquer the other. However, as long as you believe in the light, the darkness can be kept at bay. Perhaps that is what she should do? Keep the darkness at bay rather than destroying it? At this point in time, I think such ideas are resonating with Aqua because she smiled after watching Kairi and her grandmother walk away. She comments on how meeting Kairi was not just a coincidence. Even so, I don't believe Aqua is quite at the point where she truly realizes that light and dark exist in a balanced relationship. When she meets Terra shortly after in Radiant Garden, she is still very judgmental about his recent actions that are far from "good." She tells him that he shouldn't be putting himself so close to the darkness, clearly because she still believes that all darkness is bad and the world would be better off without it. Later, when she first hears that Terra had a hand in Eraqus' death, she is dumbfounded and can't believe it. Murdering Eraqus would be "evil." Terra giving in to the darkness would be the only explanation for such an act.
When Aqua confronts Terra at the Keyblade Graveyard, she realizes that the issue is not so simple. Yes, Xehanort played on the darkness in Terra's heart in order to help him kill Eraqus. But at the same time, Terra was only trying to protect Ven. Eraqus wanted to kill Ven so clearly saving Ven's life would be considered a "good" act. As a result, the issue of Terra killing Eraqus falls into a gray area rather than being black or white. Not all moral issues can be boiled down to being "good" or "bad." I believe that this is the turning point in Aqua's perception of light and dark. If she had not been challenged before this point by Cinderella's Fairy Godmother and by hearing Kairi's grandmother's story, I think she would have reacted a lot more strongly to what Terra did. She probably would have accused him of not trying to find another way to spare both of their lives. She would have naively clung to the idea that there would have been a better, "good" option that would have avoided death. Aqua is no longer the same naive person as when she left the Land of Departure. While she mourns over the death of her master and would by no means consider what happened a good thing, she understands that the matter falls into a gray area. In order to have an act of light, there has to be some darkness to maintain the balance. By saving Ven's life (light), Eraqus' life was sacrificed in the process (dark). To expect an ideal option all of the time is simply unreasonable.
After Aqua faces Xehanort in Terra's body, she dives into the darkness in order to save Terra. A burst of light can be seen coming from the portal of darkness after she goes through (probably symbolizing there is light even in the deepest darkness). Once she reaches him, she quickly realizes that there is no way they will both make it to the Realm of Light before the portal closes. She takes off her armor and puts Terra's body on her Keyblade to guide him back up to the light. Saving Terra from falling into darkness, however, would come at a price. The balance of light and dark would need to be maintained. After sending him up towards the portal, she knew that she would fall into the Realm of Darkness by herself in exchange. There was no wondering if there was a better option that would involve light triumphing over the darkness. She knew there was no point in naively thinking there was an ideal way to save them both. By this point in the game Aqua understands that light and darkness exist in an equilibrium and that in order for the worlds to function, the balance had to be maintained. If saving Terra required her to pay the ultimate sacrifice, that would just have to do. It wasn't pleasant, but it was the truth. Aqua is able to fall into the Realm of Darkness with no real qualms other than apologizing to Ven for needing to take longer than she planned to come wake him up. Though not a happy "ending," I do believe her sacrifice is evidence of her growth and maturity and displays that she is no longer influenced by naive thinking like she once was.