I think it made perfect sense that Arya killed the Night King. She did all that training for something big. Everyone expected Jon to be the hero, so it was terrific that it didn't turn out that obvious way. Also, Jon didn't really know what the heck he was doing in this war. Also makes perfect sense: Jon Snow still knows nothing. All major characters need to do something big in the final season. Bran & Arya have now probably done their big parts. I expect the other major characters being more proactive in the last 3 episodes.