Maisie Williams is annoyed, because she didn't read the books. She doesn't know what's in store for her character - Arya Stark.

She called book fans "snobby" and "mean" and they ruin the show for others who are still in the dark. She told TV Line:

I’m so sick of going on the internet and seeing all the book readers being snobby, spoiling it for other people, then saying, ‘Well, it’s not a spoiler. The books have been out for years.’
Like, couldn’t you just stop being mad for a second and let other people enjoy the show?
They feel they have a claim on the series because they read the books first, and I understand that, but they don’t need to be mean about it.

Then when Lady Stoneheart was left out of the TV series, she said this:

I liked moments like this, because book readers think they know what’s coming, then we change it and it’s really funny to watch their reactions.
They’re always like, ‘That’s not what happened in the books, so the show’s really bad now.’ But really, they just feel insecure because they’re used to knowing what’s coming next.

So don't spoil the show for people who don't know what's coming. Heck, even you might not know what's coming.