In one of my psychology classes, we were talking about how violence in cartoons and media affects aggression in children. One of the studies we looked at evidenced that children are more likely to identify with the "hero," and the violence that hero characters exhibit are more likely to increase aggression than if the villains are violent.
This made me think of how Aang is a pacifist (which he makes clear both in word and action), and how hesitant he is to kill enemies, especially Ozai. He's the hero of the story, the one kids are most likely to identify with, and he's the least violent person in the whole show. He even sometimes tries to make friends with his enemies, such as Zuko when he was the Blue Spirit.
Contrasting that, the villains are quite merciless and brutal. Azula in particular has no qualms with destroying other people. The first scene where she is introduced, she is threatening the life of her own men (verbal violence), and later almost kills Aang with a bolt of lightning (literal violence). Even Zuko in his villain era burned down Suki's entire village.
Though there are many deeper threads narratively surrounding the pacificism vs violence, I think the psychology of how this affected kids watching the show is very interesting. Theoretically, kids who watched Avatar are far less likely to be aggressive than kids who watched shows like Power Rangers (where the hero characters are engaged in intense battles like every episode).