Arya will be the one to mercifully end Cersei’s reign and save the civilians of Westeros from certain death. Arya learned mercy from her father and was mad over his death. I can stitch together a number of reasons why my theory could play out. Many have said Arya will wear Jamie's face, but I think Cersei will die by arrow.
Arya only has one king that she follows and that is the true King of the North: Eddard Stark. Arya has remained loyal to her family and never stopped serving her father as his Hand, despite Ned's death. The same way Ned protected Lyanna Stark after her death. What follows is my detailed theory that includes the prophecies of Melisandre and Maggy the Frog. Arya will kill the green eyed Cersei as foretold by Melisandre. But, Cersei will die at the hand of the valonqar, Jamie. How these prophecies weave together is really quite simple needlework. Let’s begin:
Keep the momentum of this Anti-Trump movement going! We may have been crying on our couches all winter since Election Day, but now you know you are not alone. Collective action is the only way to get to critical mass and sweetly usher these bastions of systemic oppression, unequal opportunity, corporate cronyism, scofflaws, ethical miscreants..... ad infinitum..... OUT of office!
*from my humble experience in the 80's & 90's*