It might be popular because it stays true to the essence of Aang's character. Fans who love the original show and Aang's pacifist nature would be drawn to it as it explores that side of him further.
Maybe it's a powerful jutsu or curse that doesn't eliminate the characters outright but instead forces them to find a way to adapt and survive, leading to interesting plot twists.
The 'aang doesn't kill fanfiction' could be unique in terms of the relationships it portrays. Since Aang doesn't kill, it might show how his friends and foes react to this. Maybe some friends admire his resolve, while foes might underestimate him at first. This can lead to complex relationship dynamics that are different from the typical action - based stories where killing is more common.
Most of the time, Batman refrains from killing in the comics. His mission is to rehabilitate villains rather than eliminate them. This approach makes him a symbol of hope and a hero who upholds the law without crossing the line into lethal force.
Batman typically adheres to a no-killing code, but there are rare exceptions in some darker or alternate comic universes where he has been shown to kill.
Build complex characters that have depth and growth. Their decisions and reactions drive the plot forward. Also, introduce unexpected twists to keep things interesting.