Azula cackled. She could feel the energy surge through her, as she prepared her final attack. She was grinning like a Cheshire cat, and laughing manically, as she regarded her to-be victim with hungry eyes, which were glowing brilliant gold in the relative darkness deep underground.
Zuko knew what she was going to do as soon as he heard her snicker with amusement. Immediately he leapt into a stance used when redirecting lightning, and was wholly thankful for the lessons he had received from Uncle on the matter.
Even as he looked, he could see the static energy being released from her fingertips, and her air of extreme confidence was overwhelming, and Zuko was still debating on whether or not he had made the correct decision when the blue sparks of inferno made contact with his own body, and he followed the lead of the water benders of the great northern tribe, that of the single bender of the southern one, and of his uncle, and then energy flow was incredible, and as it reached his fingertips, he was one with the current of the great surge of lightning running through him, and he looked back towards his sister, his own blood running through her veins, and all of a sudden, he couldn't do it.
As soon as he had looked her in the eye, he had known, perhaps before then, as long-lost memories of Azula as a child raced through his head: Following him around, begin for a game of hide and explode, 'feeding' the turtle ducks, practicing fire bending with him once her gift had been revealed to her and the world, and having fun. Just an innocent, care free, worriless little kid.
He had let himself become distracted, and all of a sudden he almost lost control of the lightning running through him, and a sharp zap! of pain brought him back to reality.
Coming back to his senses, he what had startled him reflected in Azula's eyes, he made the slip second decision that even if he couldn't bring himself to kill her, he must, must, MUST at least hit her somewhere that wasn't vital, for Azula wouldn't hesitate to end him, and they both knew it.
So, bringing his fingers forwards, he let the lightning fly.