Summary. It was the footman Zane, and for no coin nor quarrel but to shield a woman he loved better than his own safety; here is how I came to it.

The Reckoning. Gaunt died in the yew walk between eight and half past, struck once from behind with a garden coping-stone taken up from the low wall and set back after. That much Quill's examination gave plainly enough, and no man disputes it. The question was never how he died but who could have been standing near enough to do it while seeming to stand elsewhere. Xerxes may be put by directly: a dozen spectators had him presenting Mabel on the lawn the whole while, and a man does not strike a blow in a yew walk while a dozen pairs of eyes hold him twenty yards off. Wat is put by on the same footing — the groom at his elbow the entire evening, working the delivery cart, vouched for continuously across the very window in which Gaunt fell. Quill himself stood among the spectators throughout and was called to the body only after the alarm, which makes him a witness and nothing worse. None of the three was ever alone with Gaunt; all three were seen elsewhere by people with no reason to lie for them.

That left the hat. A footman's hat, in a walk where no footman had business, and a peg in the servants' hall standing empty of the very livery that should have hung there. The household keeps two footmen who are, from behind or at any distance greater than a handshake, one footman — Zane and his twin sister Fifi, got up alike and thought a pleasant curiosity by everyone who has never had cause to look closer. A spectator saw a footman near the yew walk and, a breath later, saw what she took for the same man at the front steps handing round a tray. She was not lying and she was not mistaken; she had simply seen two people and called them one, which was precisely the arrangement Zane and Fifi had built between them. While Fifi carried trays and lit lamps at the front, seen and remarked upon and answering to her brother's name, Zane went down from the front steps into the yew walk, took up the loose coping-stone, and struck Gaunt once from behind. He was back at the front before the display ended, and the stone was back in its gap in the wall.

As for why: Gaunt had spent his last afternoon closeted with his half-sister Lavinia, and had come out of that room speaking of disgrace and of setting a wrong to rights, which in a trustee's mouth is not idle talk. What the maid overheard through the door was the sound of a threat being made good. Lavinia had for some months kept an attachment of long standing with the footman Zane, of a kind the household did not discuss and the servants, to their credit, did not gossip of even among themselves. Gaunt had learned of it, and being an unbribable and severe man, meant to expose it — which would have ruined his sister outright and left the twins, who had nothing else to stand on but their situations, with nothing at all. Zane did not kill for money, nor out of any quarrel of his own with the dead man. He killed to stop a thing from being said aloud that could not afterward be unsaid, and he built his sister's resemblance into an alibi to do it. It is a shabby motive dressed as a tender one, or perhaps the reverse; either way it answers the case. I have charged him accordingly. This is the third such matter I have carried out of that house this twelvemonth, by my count, and I confess I no longer come to it hoping for a quiet evening.

When all's said and counted, the deed it was done,
and Zane the answer, when answer was none.
Poor Gaunt 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.