Thanks to Rhyolight04 for the beta, I couldn't have done this without her :)
He's always thought that she would go out fighting an impossible fight, saving the world in a blaze of glory, dying the ultimate heroic death owed to her. He's never imagined the sight before him now: her lying in a bed, ancient hands folded across her chest, struggling for every raspy breath. She's dying, and not from wounds inflicted by a great and powerful enemy.
Just from age.
It was so hard, every day, watching her get older, while he simply stayed the same. It was as though he was set out of her time, frozen in the moment while her clock continued to run down. Grey streaks ran rampant in her wispy hair while his was still immaculate; the day when she could no longer hold her hand steady no matter how hard she tried, he was still as ageless as ever. His beloved aged and withered before his eyes, as he watched with his endless youth.
He's offered so many times to turn her, to find something that would make her immortal along with him; but everything that would take away even a fraction of her humanity she refused. She wanted to be human. He never thought she'd have a human death. She should have died for a reason, for a cause, for something. Not simply because it's what happens to everybody.
Now she lies motionless, unable to control her movements; he has her under sedation rather than watch the trembling. Every breath she fights to take hisses through her lips like they are sandpaper, and the constant scratching and whistling fill the room and echo farther down the corridors than seems possible. He reaches out and takes her hand, hating himself for hating the feel of the papery skin, thin and sagging over the shrunken valleys between her tendons.
Everything they did together now seems insignificant; everything they had has fallen so far behind them that he can't touch or see it anymore, and she is beyond remembering even her name. She's forgotten everything that happened to her in her overly-long life, and in some cases it's a blessing; he knows only a fraction of the tortures and agonies she's suffered at the hands of so many. But when she started forgetting things he'd told her only the previous day; then the previous hour; then only a few minutes before… She disappeared, piece by piece, while he remained static. It wouldn't be so bad if they grew old together, the way they had done everything else together. If they could stand side-by-side and face the coming dark quiet together. But they can't. The quiet has already claimed her, and he is helpless but to watch the dark coming closer and closer – but only for her.
He cannot do this. He cannot stand by for what could be eternity without her. He cannot stand here another day, watching her die more and more by the hour. There's nothing left for him here. She was gone a long time ago; it has been weeks since she was last awake, months since she knew who she was. Everything they shared is locked away in his mind, and totally eradicated from hers, just footnotes in the distant, misty life with each other. When she wasn't just a shroud around a living death.
He's never done anything cowardly in his life with her, not once. Never backed down from a fight because he was afraid, never turned away from the inevitable. He's always stood tall, taking everything life – and death – had to throw at him. But he can't do this. Not anymore. He can't – he won't – watch her die anymore. He's just not strong enough. She's worth it – she should have that much – but he can't. He can't watch her die. Agony tears through him every time she breathes, knowing this breath might be her last. Knowing that following this exhale, there might be only silence. It's selfish of him to think of his pain, when she's the one dying, but even though she has no clue who he is now, he knows that when she was still healthy she wouldn't have wanted him to suffer like this. Not for anything, and certainly not because of her. He takes comfort from that, trying to imagine the young woman standing beside him, even as he regards the empty shell in the bed. They are both waiting for death, and the waiting is unbearable.
She doesn't deserve this; she should have some way out, but she promised him a long time ago that she would face death on her own terms. He knows how easy it would be to end her suffering, her torment; give her peace at last, end the constant waiting and give her the end that is coming anyway. But he can't. He wants to, sometimes; knowing the damage done by time is irreparable, knowing that inside, she is already gone. But she promised that she would die her way, and even though she can't remember making that promise, he's going to make sure she keeps it. She will die a human death, her own death, having refused all the alternatives that would make her less human. But he won't die. Not from age, not from anything. Yet he can't live without her, and he can't watch her die.
As Spike lifts the stake to his chest, his last thought is that he's glad Buffy is so far gone that she won't have to watch him die either.