Summary. It was Aziz, the companion, and he did it to keep her from Xerxes and from the truth she meant to have of him; here is how I came to it.

The Reckoning. The room and its darkness prove nothing beyond what we already knew: that a company facing a lit piano for half an hour will see the piano and each other's shapes and nothing else. What matters is who could not have been elsewhere. The Colonel sat the whole window in the front row, remarked upon the Chopin loud enough to be remembered, and was in Cousin R's sight and Dr Quill's from nine until the lamps came; a man cannot strangle a woman in an ante-room while being watched approving of Chopin in a drawing-room. His grievance, in any case, was no grievance at all — he paid Lavinia four guineas a week with his eyes open and said so to anyone who raised a brow, and the sum stands entered in his own book, unashamed. Cousin R sat the same window in the same two men's view, and whatever he is — a collector of other people's arrangements, a man who prices what he watches — he had set down in his own hand a fortnight before that he hoped Lavinia's engagements would continue, not end; there is no profit to him in stopping a trade he meant to keep watching. Fifi never left the servants' hall, on Mrs Gow's word, and her quarrel with the dead woman was a shawl, aired a week since to the amusement of the kitchen — a poor seed for a strangling. The Captain, the footman, and the vicar were all elsewhere and well attended in their elsewhere-ness. That left the back stair, and the man Mrs Gow saw go up it at five past nine and come down at twenty past, with his coat over his arm and nothing to say for the quarter hour between.

Aziz has no standing in that household but what Xerxes gives him — no property, no family in Cairnholm, no name of his own that opens a door without Xerxes behind it. What he is to Xerxes, beyond companion, is not a matter I propose to set down in a police file; it is his own affair, and none of the law's, except that it is the whole of what he had to lose, and he had twice before shown he would go too far to protect it — a drunk on the Shore Road, a creditor at the door, both handled with a violence well past what the trouble warranted. Lavinia's trade was reading what a room declines to say, and at the Friday sitting she had said, plainly and in her own voice, that there was a locked door in that company and a man standing in front of it, and that she would have the name of what lay behind it inside the month. On the Saturday she wrote privately to Aziz asking to sit with him alone before Sunday — the note was found scorched but unburnt in the ante-room grate, which tells me he read it and could not bring himself to finish the burning. When she rose from her chair in the dark and walked toward the ante-room, she was walking toward the passage where Xerxes lay dosed and dozing, defenceless, on the far side of the only door between them. Aziz met her there, cut the cord from the window with his own knife, put it round her throat, and laid her across that doorway — not to hide her, for nothing about the arrangement hides anything, but to bar it. The knife folded on the mantel is a tidy man's habit, not a killer's cunning; the placement of the body is not concealment, it is a barricade. He was never protecting himself from exposure of his fraud, for he had none; he was protecting the man asleep beyond the door, and the roof and the name and the whole quiet arrangement of his life, which amounted, in the end, to the same thing. It was overreach again, the third time this twelvemonth by my count, and this time past mending. I have charged him, and the file is closed.

When all's said and counted, the deed it was done,
and Aziz the answer, when answer was none.
Poor Lavinia lies quiet, a candle, a knife,
the tidiest exit from Cairnholm and life.
The lesson is here, should we trouble to learn —
but the cards, being cards, will be dealt a fresh turn.