Arya is one of the most complex characters because she goes from being a sweet, brave tomboy to becoming a stone cold killing machine to rediscovering her humanity.
Remember, in the beginning of season 7, she’s on her way to kill Cersei and hears that Jon Snow has retaken Winterfell. Her love for him is the one trace of humanity left in her, and the power of that love makes her completely change direction. This is the point where the switch gets flipped.
After that she reuintes with Sansa, and finds some closure in killing Littlefinger, the man most responsible for the death of her father and the wars that destroyed most of her family.
The scenes with the Hound before the Battle of Winterfell, where she says she didn’t want to spend the last hours of her life with those “miserable shits” (Beric and the Hound) and the decision to seek out the one boy with whom she wanted to become family (back in Season 3) and experience love is all part of her arc returning to life.
While I hated the idea of her being the one to kill the Night King, and ending all the interesting story lines and prophesies, as a past disciple of the God of Death, she was well prepared to do the job.
I think the ride back to King’s Landing to finish the list was still important, but more like a habit being played out after so many years. With all the other actions pointing her back toward life, her motivations were far more conflicted and the decision to walk away was not as mind numbingly stupid as Jamie abandoning his entire path of redemption to be with Cersei.