Out of the corner of his eye, Sozin watched his brother fade into the distance and disappear. He leapt to his feet and bolted to the palace library, pawing through the firebending scrolls until he found the one he was looking for. It was a high-level technique - one which, given his mediocre training, should have been way beyond his skill level. It was also lethal.
Slipping the scroll into his robes, he silently padded through the halls to the guards' quarters, and snuck into their training room. It was deserted, as any guards not currently on duty were outside running drills. Sozin sat on the floor and unfurled the scroll, studying it intently. He had a natural gift for firebending, but he was still attempting a highly dangerous move. If he didn't execute it just right he could end up killing himself instead. He pored over the scroll, memorizing the stances, visualizing the movements in his head. Then he stood and slowly worked through the motions without using fire. As he quickened his pace and felt more comfortable with the moves, he began using his firebending. It was nearly dusk when he felt confident that he had perfected the technique. He hurriedly rolled up the scroll and returned it to its place in the library, and returned to his room to wait for dinner, and his brother's return.
The evening meal was uneventful, all attentions focused on Kozen as usual. Sozin simply smiled and politely polished off the food in front of him, before excusing himself and heading towards the courtyard. Kozen raised an eyebrow, wondering what his brother could possibly find interesting outside at night. Once his plate was clear, he followed his twin brother outside.
"Sozin?" he asked quietly. "What are you doing out here at this hour?"
"Oh, I'm glad you came, Kozen! I want to show you something." A faint smile crept across his face, invisible in the dark. Sozin had meticulously planned every minute, down to the last detail. The darkness would offer him cover so his well-trained brother couldn't detect the movements he was performing and anticipate the attack. It would also provide a convenient excuse for his "mistake" when the adults came to investigate what happened.
"My firebending teacher is a total idiot, so I've been going through the library and teaching myself some moves," Sozin continued, producing a harmless-looking pair of fire daggers in his fists. "Pretty sweet, huh?"
Kozen smiled, impressed by his brother's ingenuity. "Wow, yeah! That's pretty good! How long did it take you to learn that?"
"Oh, you know," Sozin stalled, adding some useless bits of fluff to the routine to put his brother off-guard. "It wasn't too hard... took me most of the day, I think..."
"Heh, I was wondering where you'd run off to," Kozen chuckled. Sozin caught him relaxing his shoulders, and took the opportunity to strike. In the darkness, Kozen could barely detect that Sozin was suddenly moving towards him, much less make out the postures and motions he was assuming. A mere few feet from Kozen's face, Sozin produced a rolling wall of fire that engulfed his brother. As Kozen opened his mouth to scream, the flames were sucked down his throat, cooking his esophagus and lungs. The flames quickly evaporated, and the boy dropped to the ground, lifeless. Sozin grinned to himself before feigning shock and horror.
"Mom! MOM!!! Something happened to Kozen!!" he cried, as his parents came running out into the courtyard. His mother collapsed next to Kozen's body, clutching him to her chest. His father could only stare at his worst nightmare come to life. He turned to Sozin.
"What happened here?" he asked softly. Sozin turned on the waterworks.
"I... I dunno, Dad! I was just showing him this firebending move I learned, and I guess I messed something up or something because I lost my balance and fell and there was all this fire and... and..." Sozin fell to his knees, sobbing. His father hesitantly kneeled beside him, placing one arm around his shoulders. "It was an accident, Dad," Sozin wept. "It was an accident... it was an accident..."
Sozin could hardly contain his glee at Kozen's funeral a few days later. The sage's words ran together in his head, all meaningless drivel that, as near as he could figure, people were required to say at funerals. Kozen's life isn't important anymore, he told himself. What's important is that I'M the heir to the throne now.
No one noticed the grin of smug satisfaction spreading across the new Crown Prince's face.