Please help me understand some of the things at the end of the show?

Hello, I am having a lot of trouble coming to terms with the end of the show. It feels like the work up to a lot of characters came to nothing. Like what was even the point of the show.

  1. Why was Arya the one to kill the Night King instead of Jon Snow? When it came to the Night King, the entire series was building tension between Jon and the Night King, only to have Arya, who had never even previously had anything to do with the Night King, only to come out and kill the Night King? Please help me understand. It felt to me like Arya's storyline would have led her to kill Cersei...

  2. Which brings me to Cersei an Jaime. It felt like Jaime was redeeming himself through the series and found some redemption with Brianne, only to come back to Cersei and die? I don't get it.

  3. How does Dany go from being savior of the people for 99% of the series to "let's kill all these innocent fuckers?" Then, even after everything, how can Jon kill Dany? Yeah she turned evil which didn't make sense, but even so, the whole thing of Game of Thrones is about power, so why would Jon kill Dany only to not take the throne himself (because he is the "rightful heir" blah blah) but to only be like "I don't want it, instead, I'm going to go off into the wilderness like a dipshit."

It feels to me like this could have been the greatest show ever made, but then the final season happened. Did the writers die right before the final season and then HBO decided to have a 7 year old write the ending? Like... I DON'T FUCKING UNDERSTAND...