After analyzing each episode, the house tour on YGTDIT, the family portrait, and basically thinking like a writer (What would I do, if I were writing this?)...
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I think that Dr. Montgomery was so grief-stricken by, and already possessing questionable sanity before, his child's death that he made a bargain that in exchange for returning his son, whatever souls departed on the property henceforth would remain captive. I think that the Infantata caused Dr. and Mrs. Montgomery's deaths. Perhaps they thought their newly revived child would be the same as before, and weren't on their guard as they should have been. Perhaps the Infantata is now the means of acquiring new souls for the house, which is why he has endured so long there.
I believe that Tate seems more calm and good-hearted around Violet because as Moira put it to Vivien, women see the soul of a person, whereas men see what they want to see (in relation to how Ben and Vivien see different people when they look at Moira). Violet sees Tate the way he was or would have been before the house took hold and began to influence him.
I am curious to see if either of Violet's apparent suicide attempts (did she just daydream about cutting her throat?) were successful.
Has anyone else noticed in the opening credits the person standing with a set of garden shears in their hands, and then shortly afterward the same set of shears can be seen lying in a chair, covered in blood? I'm wondering if there are any clues hidden in these opening scenes?