Free Kill the Dead by Tanith Lee

could inconvenience her now. She smiled radiantly at him.
“No. See, my hair’s quite dry, and my dress. But you’re wet through.”
He let her take him to the door, and he opened it Not because he cared about the rain, but because she had seemed to want them to go inside.
They wandered about the benches and the chests. The books were piled untidily and the slates more so. A mouse pranced over the tiles. It had been eating the large candle which the tutor used to tell the time. The atmosphere was very dark, yet somehow Parl could see everything well. Even when the girl hurried up the narrow stair to the attic, he was able to follow her with ease.
The floor of the attic, which rested on the beams of the hall below, was mainly rotten from the leaking roof where the rain even now entered, and where the sprays of winter ice would poke through to drip slowly on the pupils’ heads fifty feet below. The joists had long since cracked. The walls bulged. The pupils were forbidden to enter the attic.
Silky ran daintily over the unsafe floor. Old parchment and cobwebs lay about. Where Silky’s feet passed over them they left no imprint.
At his first step after her, a plank groaned. At the second, he heard the wood crack quietly. In that instant, he was aware of how she invited him and where, and it did not matter. There was a savage sweetness in her face, pain that she would cause him pain, happiness, blind and foolish, that called for him to come to her. If she saw anything, it was their life together–their unlife –children and lovers, wedded forever in the shadows.
Then his foot went through the rotten boards as, years later, most of his body would go through the rotted struts of a bridge.
The escaping manoeuvre was complex and almost hopeless, but somehow he achieved it, flinging himself away from the floor, and from her. He landed in the doorway in a shower of splinters. His head rang, and he heard her through the ringing, murmuring to him, coaxing him to return.
When he could look at her again, she was still smiling. She held out her hands, mutely encouraging him. A moment of discomfort, and all would be well. A moment, only a moment.
He staggered down the stair, and back into the school room. He was not certain what he meant to do, but, as if it had been planned, his confused gaze settled instantly on the tall wax time candle, and the flint and tinder that lay beside it.
He did not know–how could he?–that the ultimate act must be performed in their sight. Yet his instinct knew, that seventh sense which would make him what he was to become, that seventh sense which all that frightful day had been forming inside him, brain and soul.
When she drifted down the stair, he already had the candle alight. She glanced at it wonderingly, then took up a slate and a scrap of chalk. He was not amazed that she could hold them in her unreal hands, the shock came when she showed him what she had written. Not that he could read it He would have needed a reflective surface for that. For, in the way of her kind, she had written unhesitatingly from right to left, back to front, in mirror writing. If he had needed any further sign, she had supplied it.
When he drew the packet of her hair from his belt, her eyes and mouth widened in frightful demented shapes. He had his first glimpse into hell, then, as the first of the great white moths dashed itself against him, throwing the filaments of its wings over his face, tearing him with the shards of its nails and its frantic unhuman cries–
The burning packet of hair fell from the candle onto the tiles.
And as he destroyed her, in that minute he learned, and learned forever, that yes, it could be possible, and essential, and unbearably horrible, to kill the dead.
It was his very last lesson in that schoolroom, as it was his last night in that town or on that stretch of land.
When the rainwater, dripping through from above, quenched the smouldering ashes, he ran away into the undergrowth of night.