She used to dream she was a princess, when she was much, much younger. She used to twirl around the house wearing her mom's old skirts and blouses, singing quietly to herself and picturing a tall, brown haired man standing in the door, waiting to take her away to his castle.
This is different, so different. For one she's older, much older. And wiser, more experienced, more aware of everything around her. She's also about to die. She hadn't ever really thought about death, not even after being told she was the slayer. She had always known, what with her being the slayer that she could die. It could happen at any time.
She was scared, more than she'd like to admit. Her heart had stopped beating when she had walked in on Angel and Giles in the library, when she had found them discussion her death. She had resisted comfort, needing to be alone, needing to let the fact sink in.
It was there, written down and there was nothing she could do about it. She could quit, but she knew she couldn't, and she hadn't meant it when she had said she would.
If she was going to die, she was taking the Master with her; she wasn't going to die in vain, leaving everything she loved behind and leaving the Master to turn the world into a living hell. She would rather die knowing everything was safe and the Master was dead with her than die knowing she had failed.