Anakin Skywalker fought a Sith Lord on a great big spaceship, stood holding a lightsaber over his disarmed (literally had his hands chopped off) opponent while Palpatine told him to kill him, and he obeyed, which started his fall to the Dark Side.
Twenty-three years later, his own son fights him on a great big spaceship, cuts off his hand, and stands over him holding a lightsaber while Palapatine tells him to deliver the killing blow. Vader is now where Count Dooku was, sitting helpless while he waits for a Jedi to kill him at the word of a Sith. To Vader, it had to seem almost like justice. This is how the story goes--Sidious takes an apprentice, then orders a Jedi to kill them, and the Jedi becomes the new Sith apprentice. Sidious will win, because Sidious always wins, and the cycle will go on, because darkness always creates new darkness to destroy it. Luke will make the killing blow, because Vader's given him good reason to, and then Luke will become enslaved to Sidious and the dark, until the day he is inevitably betrayed and killed himself...
But then Luke tosses aside the lightsaber.
It must have been like light breaking through the darkness, to receive mercy in the situation where you yourself had no mercy. To see your son be stronger than you were, because back then you had not known that it was strength to refuse to use power. This boy who's so much like you found a way to break the cycle and refuse the chains that waited for him. The story doesn't have to end the way it always does. He found a way to defeat the darkness by refusing to give the darkness what it wants...
Anyway, it's such a perfect parallel.
