Lady Stoneheart represents revenge the way GRRM intended: limitless, destructive, unclean.

By giving this moment to Arya in a crowd cheering moment, Arya is no longer a real character on a real journey, she instead became an omniscient, fast travelling, can-do-no-wrong dora the assassin.

They removed the best twist of the books post-season 4, then ruined one of the best characters by offloading all that baggage on Arya, and then completely subverted the way revenge is meant to be portrayed as intended by GRRM.

All this... just because Stoneheart might've been a bit meaningless? Lol. At that point, they should've just removed Oberyn Martell too.