Arya didn't take long to find her way back to Westeros — and the very first thing she did when she got there was reap some bloody revenge on those who have wronged her. Arya took out Walder Frey cold and quick. But why did Arya kill Walder Frey?
Arya has wanted to kill Walder for years: his name was always on her kill list, just waiting to be crossed off. Walder is among the men responsible for the Red Wedding, which remains one of the bloodiest spectacles on a show known for its bloody spectacles. The Red Wedding took Arya's brother Robb and her mother Catelyn from her; the Freys betrayed her family and desecrated their bodies, and it was all thanks to a plot hatched between Walder, the Lannisters, and the Boltons. She's had it out for Walder Frey ever since, and with good reason.
Arya waited until festivities were over to strike, and she utilized every skill she learned from the Faceless Men to do it – but with a creative twist to her revenge that I'm pretty sure she didn't pick up from Jaqen H'ghar, unless he teaches a cooking class at the House of Black and White that I don't know about.
Walder sat alone in his hall when a serving girl handed him a slice of pie. He demanded to know where his sons were even as the girl insisted that they were here – they were both right here. She gestured at the pie, horror and tension mounting as Walder peeled back the flaky crust to reveal what could have been a finger, or a toe. Then she pulled off her face and viewers saw that the serving girl was really Arya the entire time. Just in case Walder didn't get the hint, she announced herself to him. Then she slit his throat.
It's been no secret over the years that Arya has violence on her mind; she has been training to be an assassin, after all. And there couldn't have been anyone watching who wasn't happy to see Walder get his just desserts. Still, there was something more than unsettling about Arya's pleased smirk as she watched him die. It felt like a turning point for Arya, perhaps more than other murders she has committed. Despite how justified it is, it means something big for the character: Arya isn't messing around. She doesn't forget grudges, and she follows through on her promises of vengeance. She may not be a Faceless Man in the truest sense of it, but she is an assassin. She's just following her own orders instead of someone else's.
Arya has never been someone you'd want to mess with, but she more than proved that tonight. Anyone with a name on her list would do well to sleep with one eye open.