In a novel such asThe Haunting of Hill House, the author purposely leaves out many revelations and details in order to elicit the reader's curiosity. In this way, the ultimate meaning of the story will belong to the reader, more so than to the author. Shirley Jackson is an author who always leaves the "last door" open and lets the audience bring closure to her stories.
This being said, there is no moral per se, in this story, but there is certainly a perennial theme that is embodied in both, the house and Eleanor: repression and what happens as a result of it.
The house is described in the following manner at the beginning:
...not sane, stood by itself against its hills, holding darkness within; it had stood so for eighty years and might stand for eighty more. Within, walls continued upright, bricks met neatly, floors were firm, and doors were sensibly shut; silence lay steadily against the wood and stone of Hill House, and whatever walked there, walked alone
This description clearly shows that this was once a home that served as a vessel of virtue; a model of the puritanical way of life. And yet, as it stands strong through the years, this core of puritanism that once held it strong cracked, allowing in ghouls, ghosts, and other evil presences. Now, the house is entirely possessed.
On the other hand we see Eleanor; a morbidly shy woman, trapped in the duty of caring for her mother, and just as repressed as the hill house was once when it was in its biggest splendor. Slowly, Eleanor too begins to crack, letting down her guard, and allowing for the spirits within the house to possess her and turn her into something else. Notice her famous last words as she approached the tree before committing suicide:
"I am really really doing this by myself"
and then switching nervously to
“Why am I doing this? Why don’t they stop me?”
This demonstrates her ultimate co-dependence and this is mainly the reason why she is repressed. We could argue that a moral would be that repression and puritanism is not a key to virtuosity, and that balanced life is ultimately what saves us all.