There are two key reasons all signs point to Arya. We’ve long suspected her as Cersei’s ultimate murderer because she’s been building up to this for many seasons. Cersei has been on her kill list since the beginning because she killed Arya’s father, Ned Stark (Sean Bean).
PureWow discussed this theory with Christopher Williams, English, History and a Game of Thrones tutor (yep, that’s a thing) for Varsity Tutors.
He purported, “She’s been broken down to nothing and now [after killing the Night King] she’s something. She’s got a purpose and she’s definitely got a goal. I don’t think the Night King was a part of that agenda… But I think tactically she’s been working toward killing Cersei.”
We have to agree, but there’s another detail that really drives the point home.
In season eight, episode three, when Arya and Melisandre (Carice von Houten) meet again, the Red Priestess tells her, “I know you.” Arya replies, “You said we’d meet again. You said I’d shut many eyes for good and you were right about that, too,” to which Melisandre replies, “Brown eyes, green eyes and blue eyes.” Arya darts off to kill ol’ blue eyes himself, the Night King (Vladimir Furdik) and we’re left to wonder who the brown and green eyes belong to.