One scene makes the movie truly horrifying *spoilers*

It's the scene where the main character hears her mom saying how she's finding it hard to love her, and she's crying.

After that you know deep down she loves her mom and wants her to love her back. Because of that scene we know her delusional intentions of helping her sister are from a genuine sense of loving her family.

That plus the ending scene where the mom is hugging her makes it an extremely sad ending. Her mom must've realized at that moment that the daughter had a much more serious mental issue than they ever knew. It also looks like the main character is just barely realizing what she's done as her mom's screaming.

to sum it up: main character killing her sister isn't scary ( I was expecting people to die the whole time), but the fact she actually loved her sister & mom when she did it, and she didn't realize it would kill the sister, is very disturbing.