Zuko never bent lightning.
Neither did Aang, for that matter.
At least, they never bent lightning the way the rest of Zuko's family did it.
Oh sure, he and Aang taught firebenders how to redirect lightning to the Ba Sing Se and back, but neither he nor the Avatar ever truly generated it.
And yes, Zuko was sure that at some point in his life, he must have reached the peace of mind necessary to be able to produce it himself. And yes, admittedly, twice, alone in one of the many private training areas the Fire Lord was allowed, Zuko had started to go through the motions to see if he could. To see if he really was as strong a firebender as Uncle, as Azula, as Fa- as Ozai.
But he never went through with it.
He'd begin separating the energies, feel the power start to build the same way it had when he'd first tried it again and again on those cliffs in the Earth Kingdom–
And he'd remember watching Ozai, watching Azula, watching them send lightning his way, Katara's way.
He'd remember the blank, blind terror he'd glimpsed on Katara's face as he ran to dive in front of her, death headed her way and knowing there was nothing she could do to stop it. He'd remember Azula above him on the ship, the fear that froze him to the deck as electricity circled around her, until Uncle grabbed her wrist and sent her lightning crashing into the rocks.
He'd remember the feeling of Ozai's lightning flowing through him, the weight of the wild, thrumming power, and recognizing how easy it would be to kill with only a few tendrils of that energy.
He'd remember the pain of Azula's lightning coursing through him when he failed to redirect it, boiling agony surging outward from his stomach, his limbs numb and twitching beyond control.
–And the scar on his chest would ache and burn, as his other scar had when he'd first tried to firebend after Father- Ozai had burned him–
And he'd let the energy go.
It exploded in his face both times, of course, just as it had during all his attempts with Uncle. Thankfully, there was a reason he practised alone in those moments, so no one was around to watch the Fire Lord blast himself onto his ass.
But there was a difference between couldn't bend lightning, and wouldn't. Zuko thought he could now. He didn't think he ever would.
Lightning killed. That's all the move was good for. It wasn't like fire – fire was warmth and life and he needed to firebend. It was in him as surely as the sun rose and fell. It wasn't like after the Agni Kai, when Zuko had to teach himself to bend again without panicking, without flinching, without gagging, when he would hold fire in his hand incrementally closer and closer to his face, alone in his cabin where Uncle wasn't around to yell at him for doing something so stupid, until he was no longer afraid of his own element.
Scared and hurt as he had been, the fire was still his, and it had felt like home.
But he didn't need lightning, or its wild power. Lightning was only death. And unless a target was lucky enough to dodge, or a strong enough firebender to redirect the bolt, there was no escaping its killing blow.
Zuko didn't want to use a technique whose sole point was to kill.
He didn't want his people knowing he used a technique whose sole point was to kill.
He didn't want his people thinking he was just like Ozai.
(He knew most of his people would actually prefer if he were more like Ozai – so many of his people who still only saw Sozin's way, Azulon's way, Ozai's way. So many of his nobles, ministers, and generals who rose to power under Azulon's and Ozai's violence and cruelty. So many peasants who saw power and brutality as strength.)
(But there were servants who didn't want to be beaten, burned, or banished for their mistakes, let alone minor slip-ups. Peasants who were happy to no longer have their children sent off to fight and their lands used for the war effort. Nobles who didn't enjoy the ruthlessness of their fellows, who kept quiet out of fear, rather than indifference. Soldiers who were grateful to be home, who instead of baying for blood of the earthbenders, were simply tired of the carnage and death on both sides.)
(It was for those people that Zuko didn't collapse under the work, under the despair that nothing would ever change. It was for them that he needed to be as unlike Ozai as possible, as Uncle had been with him, and hope the rest would follow.)
It was that desire to defy his father's and forefather's legacy that drove him to teach lightning-redirection to any firebenders willing, from nobility to peasantry, inviting Aang to join him when he could. Because Fire Lords didn't teach. If they did, they didn't teach anyone but the highest of nobility. They didn't teach defensive moves. They most certainly didn't teach with the Avatar.
And Zuko was happy enough ignore each and every one of those traditions, holding classes twice a year until the technique was known far and wide. And Aang was happy enough to join him, all smiles and boundless energy for a Nation who was still learning to not see the Avatar as the enemy.
(And sometimes, sometimes Uncle would make his way from Ba Sing Se, happy enough to help them. He would explain redirection in a way Zuko never could, standing by Zuko's side while they taught as if Zuko was anywhere near his equal in bending. Afterwards, Uncle would tell Zuko how proud he was, that he was glad Zuko was passing his technique along. And for a moment, Zuko could feel proud of himself as well.)
Zuko never taught anyone how to bend lightning, though.
It was while he and Aang taught a fresh group of firebenders to redirect lightning, ten years after they won the war, that Zuko realized Aang hadn't even learned how to bend lightning. As Aang's firebending Sifu, that should have been Zuko's job. Yet as soon as he had the thought, Zuko knew he didn't need to ask Aang if he wanted to learn. Zuko knew Aang wouldn't use it.
It wasn't just because of memory of the pain they shared – both starkly reminded whenever Aang decided he needed a brush-up on training from “Fire Lord Hotman, Your Royal Fieriness”, and the two would spar shirtless, Azula's starburst scars matching his chest and Aang's back.
It was that if Aang wouldn't even kill Ozai, lightning was out of the question.
Aang told him once that in his battle with Ozai, Aang had caught the lightning Ozai had shot his way. He had redirected it the way Zuko taught him, aimed the lightning right back at Ozai...until he saw the fear on Ozai's face. Like Zuko, Aang couldn't hit him. He couldn't take a life.
Zuko had never seen Ozai afraid. He couldn't even imagine it. The few times Zuko had visited him in prison, he had seen Ozai as the barest shadow of himself, he had seen bitterness, hatred, even brokenness. But never fear.
After Zuko had sent Ozai's lightning crashing at his feet, Ozai must have realized what lightning redirection could do. That it could turn the death he meant for someone else back on him, that not even he, filled with all the power of the Comet, could prevent it.
(Zuko didn't know if he thought Aang lucky for seeing that fear, or if it awakened some small, scared part in himself that still saw his father as invincible, as a terrifying figure towering over him, a fist full of fire drawing closer to his face. Either way, it unnerved him.)
No, Aang never asked to learn how to bend lightning. In return, Zuko never offered to teach him.
And though Zuko didn't begrudge Izumi, and then his grandchildren, for learning, Zuko never bent lightning himself.
At least, not traditionally.
He just had to wait for electricity to come along first.
Electricity took a while to wend its way into the palace, and even then Zuko only allowed the newfangled electric lights installed in his office because they were a gift from Republic City. He much preferred a good torch, but it would have been rude to refuse the offer.
At first, he hadn't noticed the lights growing infinitesimally brighter when he tried to hold his temper in check at yet another bit of stupidity coming out of the United Republic, or his ministers bungling some diplomatic issue with the Earth Kingdom. Nor had he paid attention to the faint buzz of power that ran through the lights, nowhere near as palpable as fire's energy, but still present.
Zuko didn't realize anything was out of the ordinary until he was in his office, ranting to Mai about some mishap or other, and took a moment to heave a large sigh...when the lights brightened and dimmed in time with his breath.
Zuko had stopped and met Mai's eyes. They both stared up at the lights, shining on innocently above them. Then Mai had given him a look, and asked dryly if they should start investing in some rubber insulation, if Zuko's early years of controlling fire were anything to go by.
It wasn't like traditional lightning-bending, the way it had been done for centuries. It wasn't even like firebending, where benders generated their own fire. It was more like the other forms of bending, taking the element from a source, and using as little or as much of it as the bender wished. Which Zuko supposed was apt enough, considering Uncle got the idea from waterbenders in the first place.
For the first months after this new revelation, he'd use his bending to brighten up a room – an exercise he found he needed more and more often as the sight in his left eye grew worse. The sight in that eye had always been poor since the Agni Kai, but it didn't start truly failing him until past middle age.
Then, one day when he couldn't stand another minute of reading reports, he decided to find out if he could redirect the electricity, like lightning.
It was so much sharper than fire, much more ready to leap out of control, impossibly thin bands of white-blue power buzzing and snapping and coiling around him as he brought the energy from one hand, down through the stomach, and up to the other hand.
He pointed at the wall and let go. It arced out of his fingers, blasting a scorch mark into the stone wall almost a meter across, and Zuko's chest ached and burned.
He tried with only the smallest of sparks after that. Until he had control.
He felt like a child again, fumbling through his firebending katas. Except so much deadlier. Except without Ozai breathing down his neck, demanding perfection, deeming him failure when he couldn't achieve his standards. Without Azula demonstrating that perfection, endlessly mocking him for his efforts.
And without Uncle praising his improvements, encouraging his attempts, correcting him where he faltered, remaining calm even while Zuko shouted and raged.
(He never thought he would miss anything about those miserable, painful years on his ship, training and training well past when he should have stopped, Uncle gently giving him instructions. Until Uncle decided it was time to gently tell Zuko to rest for the night, or at to least stop and eat something.)
(He never missed his uncle more than when he was carefully moving sparks down one arm, through his stomach, and out the other arm, imaging Uncle's voice supporting him, cautioning him, guiding him. Or when he remembered doing the motions with Uncle for hours on those cliffs in the Earth Kingdom as Uncle soothingly told him to “feel the flow”, until he thought Zuko had mastered the move.)
(Zuko was almost as old Uncle had been when he left with Zuko on his banishment, and Zuko still sometimes felt lost without him.)
It took months before he moved onto larger currents, and a year before he decided he was competent enough that any mistakes wouldn't be fatal.
He decided the best way to show off his discovery to his old friends, next time they all met together in Republic City, was to give Sokka a shock. A little shock, only a bit stronger than one would get after rubbing against wool and then touching metal. But from the pitiful sounds Sokka made, Zuko might as well have tried to bend a lightning bolt at him.
At least the rest thought it was neat. Aang wanted to learn immediately, and Toph wanted him to test it on Sokka again (though promised to pummel him if he tried it on her, diplomatic immunity or not). Katara begged him not to teach Aang, because she foresaw many days of static-y hair as Aang practised, which of course Aang didn't have to worry about.
(Drawing a bit of electricity from a lamp and showing Aang how to snap a spark between fingers, Zuko wished that Uncle could have lived long enough to see this.)
(He knew Uncle would have enjoyed exploring what lightning-redirection could do.)
(He hoped he would have been proud.)
Still, despite all his practice and demonstrations, for years lightning was little more than a tool to Zuko, or a party trick. He didn't need it, like fire. Zuko learned it mainly to have control, as all firebenders needed control, lest they burn their family or their house. He learned it so that he wouldn't electrocute anyone.
He learned it for the same reason he never bent lightning in the first place.
It wasn't until the medical students at Ba Sing Se University discovered that electricity could restart a heart that Zuko realized lightning could be something different.
Lightning could be life.
It still wasn't like fire. It was sharp and it snapped in a way fire didn't, too eager to move and rush forward when fire was content to sit in place and merrily burn away. It was all light, little warmth, and it never felt at home in him the way fire did.
But if lightning could bring life? If it could restore a heartbeat?
Then, for the first time in his life, Zuko was actually happy to use it.
No, Zuko never bent lightning traditionally. Neither did Aang, for that matter. And both of them were pleased to leave it that way.
But lightning redirection?
At that, they were old masters.