It was difficult to remember a time before he could bend the four elements, and even more difficult still to remember life before airbending. But he could, with effort, recall that there had been a time when the tiny heat of his firebending did not exist in himself. It seemed cold and sterile, less joyful than the exuberance of flame. He could, with a little more effort, imagine not having it anymore, and were it gone he would feel less himself, less than whole.
He can't even imagine what he's put Ozai through; Ozai, the man who valued power even above the lives of his own children. He'd thought the man heartless, but more accurate, he knows now, would be to say that his heart had been set upon the wrong things; and though Aang knows (believes) this judgment is accurate, he has also come to believe (know) that the moral value of a loss has no impact on the strength of its sting.
Having felt that sting, Aang understands now that to show mercy is not always to spare a life. He regrets that he did not understand this fifteen years ago.
But there is no way to kill Ozai now, and so he lives with a mark on his conscience borne of a deed that is worse than murder, and visits the man every year in a small, inadequate act of penance.
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