In 1983's Return Of The Jedi, which turns 35 this month, the narrative, thematic and emotional consequences of that decision reached their resonant peak.
The third Star Wars film to hit cinemas might be best remembered these days for Ewoks, Jabba the Hutt, and Admiral Ackbar exclaiming: 'It's a trap!'
But it's the showdown between father and son near the movie's climax that is Jedi's finest achievement - specifically a stirring 52 seconds of action that combines visceral fight choreography, genuine poignancy, and a majestic crescendo of orchestral music.
The final duel
At a time when blockbuster film series - and successful ones at that - were still a rarity in Hollywood, Return of the Jedi was the climax of an epic character arc that had begun six years earlier with the influential first Star Wars film, 'A New Hope'.
1980's Empire Strikes Back added layers and depth to the journey of galactic saviour Luke Skywalker. Teasing his anxiety that he could slip to the dark side like Vader, and then - of course - that famed revelation: 'I am your father'.
Yet while Empire is often the most celebrated Star Wars film, it was Jedi that brought the arc to its conclusion.
Near the end of the movie, things are desperate. The rebel fleet has been lured into a trap, and are being destroyed. Luke, frantic to save his father and lure him back to the light, finds himself battling his nemesis in an intense duel (while the evil Emperor cackles, and delivers delicious, withering lines).
Luke hides in a shadowy corner, refusing to fight - so Vader taunts him. He has deduced that Luke has a twin sister, Leia. And if he will not turn to the dark side, "perhaps she will".
At that moment Luke, who we have come to see as the calm, noble embodiment of virtuous good and of hope, finally loses control.
Watch it below:
The power of John Williams
It is the culmination of everything that has come before. Driven to fury out of a desire to protect the ones he loves, Luke unleashes a torrent of pent-up rage and emotion - the duel devolving into an unhinged onslaught of hacking, slashing blows.
This results in him brutally severing his father's hand; a reversal of misfortunes from Empire.
As he steps back from his actions, delirious and panting, the Emperor's laughter echoing in his ears, Luke is forced to confront his worst fear - that he could, and maybe even has, become his father.
He has finally 'defeated' Vader. But the triumph is tinged with tragedy.
Thematically, it's a key moment. And John Williams' music mirrors the outburst of emotion - suddenly surging from ominous brooding to a soaring swell of strings and choirs.
'Final Duel (Jedi's Fury)' is one of Williams' most beautiful, powerful, yet often overlooked movie compositions; a near minute of splendour which you can listen to below in all its isolated glory:
As Luke regards the injured Vader, and looks to his own twisted robotic hand, we see the realisation dawn. How close he has come to falling into that same abyss. When he steps back from it, and tosses away his lightsaber, it's his defining act.
A family affair
At its core, the original Star Wars trilogy is a simple story of good and evil. A sci-fi fairytale of space cowboys and wizards, given heft by then groundbreaking effects, stupendous sound design and music, and charismatic performances.
But at its apex, the Luke/Vader conundrum reaches a genuine moment of power and pathos for those who have followed the saga from the beginning.
A moment which renders Vader's ensuing redemption all the more satisfying. (The silent conflict as the camera focuses on his mask, lit by force lightning blasts, is fantastic).
In recent years, Star Wars' tendency to focus on the tale of one family - the very reason that creator George Lucas has likened it to a "soap opera" - has brought a certain amount of ridicule at its expense.
But before it got bogged down in fudged, cartoonish prequels, and the cascade of Disney sequels and spin-offs we are now witnessing, it was a straightforward but engaging tale of one young man's quest to overturn tyranny, and save his father in the process.
Return of the Jedi delivered on that concept beautifully. And no amount of moaning about the Ewoks will change that.
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This article originally appeared on our sister site, iNews.
[Main image: LucasFilm]