For: Betraying her mother and Robb at the Red Wedding.
Has he been killed yet? Yes. In the season-six finale, Arya disguises herself as a servant, serves Frey a pie with the remains of two of his sons, then shows her true face to ensure that the last thing Frey sees before he dies is a smiling Stark. That's not all! In the season seven premiere, Arya uses his face and poisons his entire family. How's that for revenge?