My belief is that the narrator, Faraday, overtly dull and rationalist, is actually the source of the evil come upon the house. He doesn't passively witness the evil happening to everyone except him - his presence is the common factor behind these events. I think he literally kills the inhabitants of the house. He is the 'little stranger'. My only question is does he know he is doing it? And is he doing it physically or psychically?
But not everyone sees it that way. For example, the Washington Post says 'Faraday calls to mind Patricia Highsmith’s clever psychopath, Tom Ripley' but the Times says 'Faraday is a decent, humane man, concerned for his patients and for the welfare of the Ayres family, but he is a dull dog, so painstaking in his narration as to raise the question of whether Waters has set herself a challenge: to mediate her story through the consciousness of a bore.'
I think the Post have got it right - Faraday is a psychopath, and an unreliable narrator. The only question is - does he know it? Is he literally and physically harming people , and reporting it (to us, the reader) as the work of ghosts, or is his subconscious producing uncanny effects (as a troubled teenager is supposed to produce poltergeists) without his being aware of it?
To my mind this extra level makes this makes the story more interesting, but it is noticeable that most reviewers have not read the story in this way. I think I would like to re-read it, with this in mind from the start.