When Aang dreams, he sees all the faces of those he has killed.
First there are the nameless, the faceless, even – rows upon rows of anonymous Fire Nation helmets, helmets that were all he could see of the armies he crushed that stood in his path. Like bugs, Toph called them once with glee, but even bugs have lives and identities, and Aang doesn't know what they are, but he wishes he did.
Then there are the faces, the ones he saw when helmets had come off with some crushing blow. Faces that are broken, or bloody, bruised from their own helmets or burned by fire that was either their own or Aang's. It's been so long now that he can't remember how each one of them died – but he'll never forget their faces.
Then there are the more recent, the ones he knows. Their numbers are always multiplying, even now – even now, no matter how careful he is, how controlled he has learned to be – even now, he cannot spare every life, even if part of him seems to shrivel and die along with each one of them.
The few that are lost are worth the many who are saved, Zuko says sometimes; Sokka puts it more bluntly: sometimes you just gotta do what has to be done, and they're right, Aang knows they're right, but he still goes among them afterwards, goes among the prisoners and the surrendered leaders, and carefully asks after the name of every person he has killed. Sometimes they laugh at him, crisis of conscience now, Avatar?, sometimes they spit in his face, sometimes they won't speak, but he takes all that as it comes, and he makes sure to learn.
At least if they have names, he can know exactly what it is he has stolen, and who he took it from.
He sees them all, his whole death tally lined up in rows behind his eyelids, and they all look him directly in the eyes before fading on to make way for the next.
And then there is Gyatso and all the other airbenders: all the skeletons he did see and those he didn't, fleshed out in the skin of his old friends and teachers. He never saw their burns, but he sees them now, gruesome decorations climbing their faces and twisting their tattoos, as though he was a witness himself – and they always remind him that he wasn't. That he ran away. That he let them die.
We need you, Aang, they chorus still, and then they fade.
And then it's his friends; it's Zuko and Toph and Suki and Sokka, though the manner of their death is always different. Sometimes their necks are broken, or their faces are sliced to ribbons with daggers of ice, or – the worst ones – they're burned beyond recognition. He never knows exactly what he did, but it's always the same: he lost control of himself, drunk on his own power or raging with it, and they were in the wrong place at the wrong time, and now they're gone.
And last of all it's Katara, the one who is always the closest to him – and always in the most danger. She won't leave him when he's dangerous, he knows it, and one day he'll hurt her, one day he'll kill her, and she'll be gone, taken out of the world by this force inside him that's never in enough control – or by him alone. He has this power, it's always there, and one day he'll get drunk on it, one day he'll decide to take over, and he'll crush everything in his path, he knows it with a sick certainty, and Katara will be last, and she'll just be gone, and – and in his dreams she already is.
He wakes up screaming, and she's there beside him, real and warm and alive, flesh and blood and arms that hold him tight and a voice that whispers to him that it's okay, that it'll all be okay, and he presses his head against her chest and listens to her heartbeat, and at least for a little while, he can believe her.
When Katara dreams, she sees all the faces of those she has lost.
Mom is first; Mom, of course, is always first. Katara sees the fear in her eyes the last time she ever saw her alive, and then she watches her body burn until it's just the charred corpse she found after, the smell of burned flesh she'll never get out of her nose.
Then it's Jet, Jet who she forgave just in time for nothing, Jet with his crumpled body and his clear-at-last eyes and his hoarse "I'll be okay," Jet who was the reason she started carrying the spirit water around with her, but who paid the ultimate price for her neglect.
And then it's all the others, from earliest to most recent: the wounded soldiers she tried so hard to heal during the war, so many of whom slipped away. The stillborn baby she delivered, whose mother looked up at her with those hauntingly empty eyes before thanking her for trying her best; the teenage boy who was wounded in a factory accident and brought to the hospital too late. Every patient who has ever passed before her who she's failed to save.
Aang is last; Aang is always last. Aang, struck by lightning, lying dead in her arms, with only one last hope for survival (and there is a tiny, tiny part of Katara that has never forgiven Zuko for that, that only comes to the surface on nights like these), and in the dreams, the spirit water never works. She bends it from its container but her healing abilities fail her, or she holds it desperately over Aang's unbeating heart and the water stubbornly refuses to be anything but water.
She wakes gasping, cheeks wet with tears, clutching at the sheets – and always he's beside her, warm and gentle and alive, letting her cry and cling to him until she knows, really knows, that he's alive. He just holds her; he never asks. Never has to. But it's enough for her just to know that he's there.
Until finally, years later, the nights come that he isn't.