Summary. It was Nan Pringle, and the reason is envy dressed as shame, and here is how I came to it.

The Reckoning. Fifi Swain did not fall by vanity's own carelessness, as the house first had it, though the house had the manner of her dying half right — she did go to that stair dressed for a meeting that mattered to her, and she did believe herself bound for something finer. She was bound only for the dark. Someone had sent her there knowing the lamp would not be, and that someone was the one person in the house who kept the cold-house key, moved unremarked between hall and belowstairs the whole evening, and had cause, of a kind the household never once suspected, to want that particular mouth shut for good.

The method wanted no strength and no weapon, only quiet. At half past nine Nan spoke to Fifi at the baize door, a thing Mrs Gow saw and thought nothing of, since Nan was speaking to half the staff that night on the business of the prizes. What she gave Fifi was the blind entry card numbered nine, as a token that the appointment was real — and Fifi, who had been pricing gowns and telling herself she was meant for better, read into it exactly the promise she wanted. She dressed, put up her hair, and walked the glass corridor toward the cold-house believing herself wanted. Between a quarter to ten and ten, while the company crowded the prize table and nobody's eye was fixed on any one person for more than a minute together, Nan lifted the lamp from its bracket at the stairhead, carried it into the cold-house, set it on a shelf, and latched the cold-house door from the hall side on her way back. A corridor glaring with hall light ends, past that latch, in a stair with no rail and no light at all. Fifi went down it in a gown and shoes she was not used to, and broke her neck on the eleventh step. Nan never touched her, and was never missing from the ordinary traffic of the house for more than ten minutes — which is why nobody missed her at all.

The others fall away on plain fact, not suspicion satisfied. Xerxes Hartfield stood at the prize table from a quarter past nine until the body was found, vouched the whole time by the Colonel and by Dr Quill, and could not have been at the stairhead and back. Captain Frew was belowstairs with two kitchen men and Mrs Gow's passing eye from nine until past ten, and has in any case no cold-house key. Lady Lavinia was in the small drawing-room with Cousin R from nine o'clock, and handled ribbons that evening, never the blind cards, so she could not have put card nine into anyone's hand. The Reverend Gaunt was in the cold-house itself, blessing it against mildew with the under-gardener at his elbow, until a quarter to ten, after which the Colonel had him in sight; the lamp was taken after he came up, not before. Zane stood at the hall door in the whole company's view from nine until the cry went up, and whatever summons drew his sister away, she had kept it from him at supper, as she kept nothing else of his.

The motive is uglier for being so small. Fifi wore Zane's own face and was desired for it, walked out with, laughed with, free to leave service by Christmas with a shipping clerk's ring on her finger if she chose. Nan is gentry by blood and has neither wage nor a door that locks, and at one-and-thirty has never once been asked anywhere. That alone might have bred only a long, swallowed bitterness, the kind a household never notices. But Nan loved the footman Zane — a thing unspeakable for a woman of her blood, and the ruin of the only property she owned, which was her own invisibility — and Fifi, who kept every other secret in the house for sixpences and told none of her brother's, had seen Nan coming up from the mews in Zane's coat and had made it a joke before two kitchen girls, and kept making it. Nan could not answer that joke with a word, because to speak of it at all was to be seen, and being seen was the one thing she could not survive. So she answered it once, in the right ear, at the baize door, and let a dark stair finish the sentence for her.

I had near forgotten the hat under the prize cloth — a stranger's billycock, salt-stained, that nobody claimed. It proved nothing against Nan and nothing for her; it belonged to no one in this matter at all, only to some tradesman's forgetfulness earlier in the day, and I set it aside as such. The case closes on the lamp, not the hat. I have charged Nan Pringle with the wilful killing of Fenella Swain, the third this twelvemonth, by my count, and the year not out yet.

Nan’s little appointment was couched with a smile,
And Fifi went dressed for a walk down the aisle.
The glass passage glittered, the stairhead was black,
The household ate pudding and asked for no lack.
The moral, we vow, is to look where we tread—
A maxim for others, but not for the dead.