I didn't know what to think of it, until I read some reviews, and then I was able to form my own.

It's true there's no context given about the sacrifices, the master, the world, the magic house and the war that goes on outside. But they aren't important because what matters here is the point of view of Charlotte, the last little girl to be sacrificed and agonizing in her grave even after death.

The story isn't meant to tell us whether they were right to sacrifice little girls for the greater good (or for greed). It's meant to tell us that no matter the reason, the sacrifice will still suffer and that their rage is valid and justified.

There are still some confusing parts, like when she kills Isaure (what was the reason? Did she herself in her? Did she remind her of herself when she agreed to be sacrificed and resented her for that?). It's unclear.