Redemption is a reductive kind of moralism. 

Zuko doesn’t really have a redemption arc, because he was never “bad.” The Diamonds don’t really have a redemption arc, because they never become “good.” Redemption is a weird external moralistic concept that has nothing to do with individual character development or lack thereof.

To put it another way: The Last Airbender never condemns Zuko, so forgiveness isn’t the point of the story; and Steven Universe never forgives the Diamonds, because nothing could ever make up for what they did.

This isn’t to say that the characters don’t change their behavior for the better. What I’m criticizing is a binary and extrinsic reading of morality in relation to narrative function, as opposed to an intrinsic reading of situational character motivation. 

Redemption is an externally imposed concept that doesn’t allow for agency or intention, but rather describes a functional narrative approach to character. It suggests: 

1. an innate change of a character’s essence, 
2. to serve the demands of another’s morality…

… which is a simplistic understanding of psychology, social dynamics, and… just, judgment. Really, redemption is all about judgment, which lies in the perspective of the narrative voice. It’s an external thing, where the story passes sentence on characters and demands that they change who they are in order to suit its morality and make up for their past sins, and to thereby be forgiven by the story. Which is a super basic concept of humanity that doesn’t apply in either case above.

Zuko is shown from pretty close to the start as a victim; he’s not a Bad Guy who Turns Good. His arc is a matter of self-realization and emergence from an abuse narrative, and its resolution involves reaching a common understanding, not repaying moral debt.

And the Diamonds, they are never forgiven. They change their behavior out of argument for how it’s not helping them achieve their own individual intentions. Even at the end, they are shown to be extremely self-centered characters who have difficulty understanding anything outside of how it affects them directly. Steven tolerates them at a stretch, once they change their behavior enough that they no longer pose a threat to others. But what they did will never be okay, no matter what they do, and the story makes no pretense of balancing the scales. 

Compare to, say, something like Buffy the Vampire Slayer, and the characters of Angel or Spike. In the case of Spike the protagonists stick a microchip in him, taking away his agency, until he gets used to behaving the way they want him to. With Angel, the change mostly happens before we meet him. But the notion is that they’re Bad characters who become Good, and then feel sorry and try to make amends for what they’ve done. Similarly Missy, in the Peter Capaldi era of Doctor Who, undergoes a redemption after serving penitence for years in solitary confinement and out of a desire to please the Doctor and try to play out his concept of morality. 

In all cases, there’s this notion of penitence and turning from Evil. With Spike the change comes after the microchip, which changes his behavior until he becomes accustomed to the new way of being, even after it’s removed. It’s a punitive, judgmental, carceral sort of a moralism. The idea is to show people how Bad they are until they are ashamed of themselves and they want to stop being Bad—”Go to your room and think about what you did”—all of which ignores the complexities of how and why people do things based on their understanding and their systemic context, and treats others as lacking a degree of agency independent of those passing judgment on them and their own individual interests.

You are not a person, the redemption narrative asserts; you are a story function within my life.