All in her head
The strange trend I've seen with The Haunting that is opposite of almost every other psychological horror film I can think of is that most people take for granted the fact that the house is haunted, or an evil entity in itself, and that it wants Nell. This has always seemed bizarre to me considering the fact that other films such as The Shining or Hour of the Wolf are always analyzed as psychological horror films, and people tend to view those films as more psychological horror films rather than ghost films. Even films such as Rosemary's Baby are analyzed like it is a dream Rosemary is having brought on by her guilt over being a lapsed Catholic, rather than a literal series of events.
I don't often see this with The Haunting. Most people I've seen discuss the film come to the conclusion that the house wants Nell and will do whatever it can to have her. I think, however, that there is a distinct possibility that any "supernatural" events that might be taking place are all in Nell's head, and that it is simply a house, albeit one with a dark past. Nell is simply not a in a good state of mind, even before she visits the house, so I think this is a plausible explanation. I'm sure multiple people have proposed this before, but I don't often see this idea being discussed, so I'm wondering what other people think.