Koh had said that Tui and La were push and pull, light and dark, good and evil, but he was not entirely correct.

La was not evil, he was simply selfish.

It is true that selfishness was at the core of all evil people. Evil people do horrible things at others' expense because it gives them pleasure, or because it gives them more power. That was selfish, but not all selfish people did that.

La was not evil, he was simply selfish.

Good and evil were not a force or an element, they were a choice. Selfishness and selflessness were forces. They lived in each creature's body equally; it was the choice of the creature which force it leaned more toward, and what it did to satisfy the urge.

La was selfish, not evil. That was why he had been sorrowful at Tui's death.

An evil thing would not have cared. An evil thing would have merely sought a new companion to fill the empty spot in its black heat, but for La that spot could never be filled. Tui had been his sister, his only companion throughout all of eternity. La would even go as far as to call her his friend.

La was selfish, not evil. That was why he had joined forces with the Avatar and eradicated all the fire nation troops in the North Pole.

He could have destroyed everything and everyone in his rage and pain, but the water tribes are his people. An evil thing would not have cared about them. An evil thing would have concord new people, taken new land, and cared nothing about duty. But La cared about his duty.

La was not evil, but he was selfish. That was why he took his revenge.

La had taken one life without the help of the Avatar. He had taken it in the cruelest way the ocean could; by drowning Zhao in his cold and lonely depths. To La it was a fitting end to the firebender, for there are two deaths that are worst of all; to burn to death, or to drown.

La was selfish, but even he could be selfless some times. That's why he had crossed the line.

La had once made a promise to himself, one where he swore to be good spirit, despite his selfishness. La kept his promises, but he had stained himself with his merciless and hateful killing of Zhoa. La had done something evil when he had made Zhoa suffer. La had broken his word, and he had done it for Tui.