The character of Merry-Kate had been unsettling since the first scene. I knew she was the one who caused the troubles, and I wasn't sorry for her getting bullied by the whole town. She believed in something unlogical, and she tried to portray this to her sister, Constance, but Constance didn't seem to buy it. Odd to see how Constance was optimistic about the world while she was the one rumored to kill her family, and at the end of the movie, we know why.
I could see that Constance was relieved with Charles around her. She started distancing herself from her control freak little sister. I did believe Charles was a good person until the fire happened. It turned out he was after the money. Also, when the aunt and uncle of Constance and Merry-Kate arrived, he left like a stranger (with fear, too).
Throughout the whole movie, I can't help but feel sorry for Constance. She must endure all the hate and took accountability for all the chaos caused by Merry-Kate. This movie is something else. It's worth watching, but the ending is just not it. I wish Merry-Kate would learn a lesson or something, but I guess we don't always have that, especially in the real world.
I don't read the book, though. I watched it for Sebastian Stan (and was not disappointed).