I think you have it right there. I don't think he himself killed Elsie, and I do think the ghost there is quite real... though we are left with some ambiguity on that point. Yet, when one considers that the ghost is, essentially, his "new" Romilly, while Elsie was the "old" Romilly, that he is, even if unconsciously, responsible for what happens to her seems quite certain. This may also have much to do with his reactions at various points to her various misadventures, including the foggy response when he hears what he thinks is her voice calling his name at the point where the ghost must have killed her. There is also the possibility that that ghost is herself (or itself) the production of his own mind... not in the sense of madness, but that some force may have been given that particular form by his own creative, artistic process concerning his novel. However, the song (suggested by the dripping of the water, which indicates an exterior... personality?... at work as well).
The following is from Jack Sullivan's Elegant Nightmares:
"Another example of this self-recoiling vision is Oliver Onions' "The Beckoning Fair One," a marvelously conceived story of a novelist who falls in love with his unfinished, ultimately murderous female creation. Onuions, like Yeats, identifies ghostly experience with the world of art, a world both ecstatic and demonic. The Beckoning Fair One represents the final triumph of the imagination, the subjective fusion of "joy" and "terror"; neither can be separated from the other, for both belong to the unified "category of absolute things." When the narrator submits to The Beckoning Fair One, he also, unfortunately, submits to the loss of his sanity, becoming, as much as the expected other ghost, a deranged spectre who lowers all the blinds and haunts his own house (much as Brydon later does in Henry James's "The Jolly Corner")."
This, on the other hand, is from The Penguin Encyclopedia of Horror and the Supernatural:
"Onions's ghostly fictiuon is characerized by psychological realism and an unusual elegance. A prime example is the superb, "The Beckoning Fair One,"a long piece from the collection Widdershins (1911). It is the story of Paul Oleron, a writer of some note, who, while working on a novel, is haunted by his own heroine. But there is more to the tale than this: in a superbly rendered blend of fantasy and reality, Oleron's personality begins to disintegrate, almost in the manner of a psychosis. His closest friend, Elsie, senses the evil in an old house Oleron rents, and is eventually murdered by the spirit haunting the place. Art itself becomes a supernatural phenomenon when Oleron submits to "the beckoning fair one," the woman of his art, who is in the "category of absolute things." This multileveled work is also a tale of psychic vampirism, as Oleron's character feeds on him and hungers for his sanity. In another sense, it is a story of gradual mental deterioration, the quintessential psychological ghost story, cited by Lovecraft, Blackwood, and others as one of the most subtle in the literature."
