Well, it could be due to a build-up of frustration and a sense of hopelessness in trying to reform the Joker. Also, maybe in that particular comic's storyline, the circumstances are extreme and Batman makes a desperate choice.
Batman typically refrains from killing the Joker in the comics. This is because Batman believes in justice, not vengeance, and killing would go against his principles. Also, keeping the Joker alive creates ongoing story arcs and challenges for Batman to overcome through non-lethal means.
Because the mystery around the Joker is part of what makes him so fascinating. His chaotic nature is better left un - explained by a traditional origin story.
No, Joker wasn't the one who killed Batman's parents. The culprit was a common criminal during a mugging incident. However, Joker has often taunted and tormented Batman about this tragic event in their confrontations.
Yes, the Joker didn't directly kill Batman's parents in the traditional sense. But his actions and presence in the Batman universe have had an indirect impact on that tragic event and its aftermath.
In most mainstream comic continuities, the Joker didn't kill Batman's parents. It was a random act of crime by ordinary criminals. But there might be alternate or non-canon versions where this could be different.
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.