Star Wars: Return of the Jedi marks the iconic conclusion to the Original Trilogy and is, at its heart, a story of good conquering evil. At the conclusion of the 1983 film, the young Jedi Luke Skywalker resists the temptation of the Dark Side and helps to defeat the Galactic Empire. Luke bests his father, Darth Vader, in combat, and then spares his life, which leads the Sith Lord to perform an act of redemption by saving his son from Emperor Palpatine. However, George Lucas originally envisioned a far more sinister end for the story -- one that definitely would not have worked.
While working on the film with co-writer Lawrence Kasdan, Lucas wasn't sure how the trilogy would end. Lucas had changed the title of the film to Revenge of the Jedi at the behest of Kasdan, who thought Return of the Jedi was too weak for the concluding installment. The version the two were working on at the time was much darker in a lot of ways. Han Solo, for example, was killed in the middle of film during a raid on an Imperial base.
However, that's far from the darkest part of Revenge of the Jedi. In the original version, Luke slew his father, removed his mask and took the mantle of Vader for himself. As the new Vader, Luke then attacked the very Rebel fleet he helped lead to Endor and went to war with his twin sister, Leia. As the movie closed, the battle between the Rebels and the Empire continued raging.
Instead of good conquering evil, Darth Sidious continued to rule over the galaxy, Luke joined the Dark Side, Anakin was never redeemed and balance never returned to the Force. There's a bit of a poetic ring to this ending, as 1980's The Empire Strikes Back alludes to Luke fearing he might fall to the Dark Side, even before he discovers that Vader is his father. When he trains on Dagobah with Yoda, Luke goes into a cave strong with the Dark Side of the Force. While inside, he's met with a vision of Darth Vader, a manifestation of his fears. At first, it seems his fear is facing Vader. But when he defeats the specter of Vader in the cave, Luke sees his own face under the mask. This suggests Luke's greatest fear is being like Vader, not facing him.
Luke's potential fall to the Dark Side is teased several other times in Empire Strikes Back. Luke repeatedly disobeys Yoda's training, and lets his fear dictate his actions twice: once when he enters the cave with his lightsaber despite Yoda telling him to leave it, and again when he leaves for Bespin to help his friends and thus abandons his training. His submission to fear is a path to the Dark Side and is practically the same path Anakin himself took to become Darth Vader. Still, that darker conclusion to the trilogy still wouldn't have worked as well as the one in Return of the Jedi.
Had Luke fallen to the Dark Side, the trilogy would have felt unresolved. Things would have essentially been back to exactly where they were at the start of 1977's A New Hope. The Emperor would still be in charge of the Empire, and he'd still have a Skywalker at his side, albeit a different one than he'd had before. There would have been no change in the world, and that would have made the story feel as if it was all for nothing, which makes for a weak ending. In contrast, the ending to Return of the Jedi that Lucas chose instead gave the trilogy the catharsis the series needed to feel complete.
During production, Lucas moved away from that dark ending, due in large part to just how dire it would have been for a franchise "for kids." And this was absolutely the right choice, as having Luke redeem his father and overthrow the Empire made for a much more satisfying ending. By having good conquer evil at the end of the trilogy, Lucas helped solidify Star Wars' Original Trilogy as the beloved set of films it is to this day.