A:TLA belongs to Bryke, not me.
A/N: This is set during the episode "The Phoenix King", between Zuko teaching Aang how to redirect lightning and the Gaang's practice for taking out the Firelord.
"Say, Zuko," Sokka said as the group ate lunch while discussing how to take down the Firelord, "I've been wondering about something you said earlier."
"What were you wondering about?" Zuko asked.
"Well," Sokka said, "you said that you wanted to speak out against your father's plan and that you're ashamed that you didn't. I was wondering why you didn't speak out against it."
"Because I learned my lesson the first time I spoke out against a cruel plan in a war meeting," Zuko said. "And it wasn't even my father's plan I spoke out against that time."
"What do you mean?" Suki asked. "What happened the first time?"
Zuko sighed. He had known that he would end up telling his friends about how he got his scar eventually, so now seemed like as good a time as any to tell them. Besides, maybe if Aang heard just how cruel Ozai could be, even to his own son, the airbender would stop having such a problem with the idea of killing the Firelord.
"I was thirteen," he said. "I wasn't supposed to be in the war meeting in the first place, but I persuaded Uncle to let me enter so that I could learn how to rule. One of the generals was proposing to use a division of new recruits as bait to draw out a dangerous battalion of earthbenders. Even though Uncle had warned me not talk during the meeting, I couldn't stop myself from protesting the plan. The general was going to sacrifice an entire division by sending them up against enemies they had no hope of defeating. I knew that was wrong and I said as much to everyone in the room."
"What happened then?" Katara asked.
"My father became very angry," Zuko said. "He said that my challenging of the general's plan was an act of complete disrespect and that it could only be settled by an Agni Kai—a fire duel. I looked at the old general and declared that I wasn't afraid to fight him, but I had misunderstood. When I turned around in the arena to face my opponent, it wasn't the general that I was facing."
"Who was it?" Toph asked.
"It was my father," Zuko said. "By speaking out against the plan in the Firelord's war room, it was the Firelord I had disrespected and therefore it was the Firelord who would be my opponent."
"What did you do?" Aang asked.
"I surrendered on the spot," Zuko said. "I begged him for mercy, saying that I only had the Fire Nation's best interests at heart. I said that I was his loyal son and abased myself. He ordered me to fight, but I refused. I couldn't fight my own father. What he said next is etched into my memory."
"What did he say?" Sokka asked.
"He said 'You will learn respect and suffering will be your teacher,'" Zuko replied. "Then he gave me this." Zuko raised a hand to his scar.
"Your own father gave you that?" Aang asked, shocked.
"Yes," Zuko said. "And then he said that by refusing to fight, I had shown shameful weakness and that I was banished and dishonored until I could capture the Avatar. That's why I was chasing you in the first place. It was the only way that I could go home again."
"That's horrible," Suki said.
"I know," Zuko replied. Turning to Aang, he said, "Now do you understand why you can't afford to show the Firelord any mercy? It's because he won't show you any, child or not. If you don't go for the kill, you'll be the one who dies."
Aang nodded in understanding, but his face was troubled and he didn't finish his lunch.