Summary. It was Xerxes Hartfield, and for no grander reason than that the man had stopped being of use to him. Here is how I came to it.

The Reckoning. The Colonel died of a broken neck and nothing else — no blood beyond a graze, no mark that the fall itself does not explain, a rucked heel of carpet and a fallen stick to write the whole business off as misadventure. That is what the scene wanted read. But a set of gentleman's prints crossed the dust of the upper landing to the head of the stair beside the Colonel's own, and the back stair from the kitchen passage reaches that same landing without ever passing the drawing-room door. Someone went up unseen. The house gave me six people to weigh against that window between a quarter past seven and ten to eight, and one by one the six came clean out of it. Nan sat in the drawing-room the whole time, under Lavinia's eye and Cousin R's. Lavinia sat with her. Cousin R sat with both. Aziz stood below in the kitchen passage asking Mrs Gow after the port, vouched throughout. Wat Frew was off the premises and down at the harbour before seven, seen so by the kitchen girl. Quill was in the dining-room decanting the Colonel's own port with a footman at his elbow — and I will note, since it is his particular engine, that the pistols the Colonel never dined without stood cased and cold on the sideboard; nothing had frightened him into reaching for them, because nothing of Quill's contriving had occurred. That left the one man in the house who owned to being about it unwatched: Xerxes Hartfield, who said himself he was seeing to venture papers and was not in the drawing-room when the Colonel fell.

The papers are the whole of it. The Colonel and Xerxes held the shipping venture jointly, each lending the other what he lacked — the Colonel his name and standing, Xerxes his capital and his trade — and a partnership built on mutual want holds only so long as the want runs both ways. That week it stopped. A syndicate's buyout, drawn and ready for signature, would hand Xerxes the trade outright. And the match the Colonel had been pressing on him, Nan Pringle's old blood set against his new money, was as good as settled — which would hand him the name as well, by marriage, and without the Colonel's continuing goodwill to hold it. A man who has the trade in his own hand and the name coming to him by his wife has no further use for the partner who once supplied both. What the Colonel had been to him — patron, guarantor, the price of respectability — is the household's own affair; what losing his use for the man drove him to is mine. He went up the back stair while the drawing-room sat occupied and the dining-room stood lit and waiting, met the Colonel at the head of his own flight, and put him down it — no blow, no struggle, a rucked carpet edge and a dropped stick left to tell a plainer story than the true one. Then, in his own words, he went in to dinner.

I make it the third this twelvemonth, by my count, and this house no stranger to my visits. I am charging Xerxes Hartfield with the wilful killing of Colonel Cecil Cruikshank, and the file closes on that.

Xerxes upon the landing turned the wheel,
The Colonel mounted stairs in zeal,
A carpet's edge became the trap,
The household took its evening nap,
The moral warns us: watch each guest—
We'll guard our stairs and know them best.