Summary. It was Cousin R, and he killed her to finish a picture; here is how I came to it.

The Reckoning. The cause was smothering, and the means her own cushion of green silk, held steady a few minutes over her face while the house was elsewhere. Quill was plain on that point, and the cushion bore it out: plumped, its nap brushed all one way, as a man brushes out the marks his hands have left. Nothing else in the alcove was disturbed, because nothing else needed to be. She had not struggled against whoever held her still, because she had not expected to need to.

The window stood open between 11:05 and 11:20, when Aziz was called to the champagne and the company drawn off to the gallery by the ices. Everyone else in this household can be put somewhere else entire. Xerxes was pouring ices in thirty people's sight from 10:50 on. Zane had the pails on the gallery stair under the butler's eye from 11:00. Gaunt, whatever his grievance over the money — and it was real, and it was already with a solicitor, which takes the heat out of it — sat in the long gallery from 11:00, three guests and a cellist swearing to it. Fifi was counting linen in company. Effie was on the dance floor till the band stopped at 11:22. Wat never left Mabel's circle, and had no business with Lavinia at all. Only Cousin R put himself out on a terrace, alone, unobserved, for exactly the span of the killing. A man may go out for air. He may not go out for air and return with the one thing nobody else in the house could have brought back.

He brought back a sketch. He had made three studies of her in that chair, he said, over three sittings — and the third shows her as she was found, head tipped back, hands folded, shawl slipped from the one shoulder, a pose she had held in none of her sittings. He had drawn the picture before the woman sat for it. That is not observation. That is intention, set down in advance and then arranged for.

The household read the thing as a family matter — Gaunt's temper over the money, or Lavinia's talk of a shut door, which some wanted to lay at Xerxes's threshold. Neither holds water, and neither was ever the true grain of it. Gaunt had opportunity in his anger but none in his whereabouts. The shut door was Lavinia's habitual mischief, said every month, and led nowhere near the conservatory. Cousin R wanted no money of her, and no silence from her. What he wanted was her ending, because he collects people at the last interesting moment life allows them, and keeps the studies, and shows no one, and sells nothing. She was the best material Cairnholm had given him in years — a woman who had spent her whole decline performing conversation with the dead — and the only way his series of her could be completed was for her to join them. He had the first two plates. He needed the last, and only she could supply it, and only once.

He has been shown his own sketchbook back, and has not troubled to deny what it says. I have charged him, and sent the portfolio to the magistrate entire, the third plate marked as the Crown will want it marked. Three murders this twelvemonth, by my count, and I don't doubt I'll be sent back to this house again before the year's done.

Cousin R left the green cushion tidily plain
While Lavinia slept through the clink of champagne
The alcove grew still as the terrace door sighed
And the household pursued its petits fours outside
We’ll vow to be watchful, and practise the art
Of leaving our lessons precisely where they start