Just some thoughts (spoilers)
I just finished watching this movie on a suggestion, and thought it was very good. It really was suspenseful because other than the letter that she had found, at times I really thought that maybe she was going insane like her husband was trying to convince her. The letter was too big of a tip off, if they didn't include it, it may have been more interesting. They could have let them find it later in the movie perhaps even convincing the watcher that maybe she had committed the murder against her aunt.
What really bothered me was how abusive he was to his wife, leading one to think that for sure it was he driving her insane. I know men "ruled the house hold" more so in those days but he was over the top keeping her prisoner and treating her the way that he did.