How different would Star Wars: The Last Jedi, Luke's lapse in judgment set the course for the entirety of the sequel trilogy. Even though Luke couldn't go through with his grim idea, his near-miss was enough to push Ben Solo to the dark side and result in the fall of the new Jedi Academy.
Despite the fact Luke didn't kill Ben, it's entirely possible that he could have. There are hints during both Luke Skywalker was not immune to the pull of the dark side; a prime example being his Force choking of Gammorean guards in Jabba's palace. While Luke dedicated himself to the pursuit of becoming a Jedi, he was a good, but flawed, man rather than a paragon of virtue.
So what would have happened if Luke killed Kylo Ren before he became Kylo Ren? How would he have fit into the events of the sequel trilogy? And what would have happened to Rey, Han, and Leia?
Why Luke Skywalker Might Have Killed Ben Solo
Luke's reasoning for nearly killing his nephew was that he could tell that Ben was feeling the pull of the dark side. As someone who had previously been a part of the battle between Obi-Wan and Darth Vader and been tasked with killing his own father, however, Luke was no stranger to the idea of taking out a family member. Of course, Luke didn't ultimately kill Vader, but an older, more disillusioned Master Skywalker may just have decided he had to do what was necessary. There are two ways Luke's fear of Ben turning to the dark side could have worked out in his mind; one in which Luke ed that he turned his father to the light against all odds, and another in which he was too afraid to take that chance again. As fear is the path to the dark side, enough fear in Luke's mind about a resurgence in Sith power could have well spurned him into executing such a horrific act.
Luke's Jedi Academy Would Still Have Failed
Although Star Wars: Revenge of the Sith.
Just like in The Last Jedi, it's also possible Luke would have decided it was time for the Jedi to end after killing Ben. Luke was noticeably hesitant to train Rey (the heir of Palpatine) for fear she would turn to the dark side, and the desire to avoid the trauma of killing another student would be more than enough to keep Luke from teaching again if he had killed Ben Solo. This is all in addition to the fact that Luke may not have been able to face Han and Leia, and thus gone on the run to avoid explaining his actions to his family.
Luke Killing Ben Would've Repeated Anakin's Dark Side Fall
But of course, Luke's actions would have implications far larger than the fall of his academy. Those moments in the original Star Wars films when Luke seemed close to the brink of the dark side would likely have coalesced after Luke murdered Ben Solo and would have set Luke on a path toward the Dark Side just like that of his father. However much Luke would have believed he was doing the right thing by killing Ben, such a horrible act committed out of fear would have almost certainly broken something in the Jedi master. Just like Anakin cutting off Mace Windu's hands to save Palpatine's life, Luke killing his nephew would have left him weaker than ever and susceptible to the dark side's influence.
Palpatine Would've Tried To Convert Or Kill Luke Again
Importantly, Palpatine was alive in some form during the time Luke nearly killed Ben. Though he was acting through Snoke, Palpatine manipulated Ben into turning to the dark side. It stands to reason that with Luke in a weakened emotional state after the act, it would be easy for Palpatine to try and convert or kill him. Palpatine's manipulation of Ben, Anakin, and potentially even Leia suggests that he would once again try to get the "son of Skywalker" on his side. While it's true that Luke had resisted Palpatine in the past, it's possible the Emperor's efforts would not be nearly as easily overcome in this scenario. It's likely, then, that Luke would have taken Kylo Ren's place in the sequel trilogy as a primary villain - which no doubt would have been a shocking reveal in the cinema.
How Rey's Story Changes Without Kylo Ren & Master Luke
If Luke were to kill Ben and fall to the dark side, Rey's story would naturally be drastically different. However, given that the Resistance would still likely be looking for Luke as they were at the beginning of the First Order could have remained largely the same. The biggest implications Luke's actions would have for Rey would have to do with the Force dyad between her and Kylo Ren, and her training in the care of Luke himself. iIt's unclear how Rey's Force power would or wouldn't have developed without Luke's tutelage. The Force Awakens introduced Rey as someone with exceptional innate Force ability, and while Luke might have helped her harness it more readily, Rey seemed able to reach for the Force successfully any time she truly needed it. This was likely due in large part to her heritage as a Palpatine.
A bigger issue would have been the absence of Rey's Force dyad with Kylo Ren, which becomes a key factor in his eventual redemption and her successful campaign against Palpatine. The dyad might have been substituted, however, for a similar (though less powerful) bond between her and Luke. The same Force ability that led Rey to the flashbacks she experienced while holding Anakin's lightsaber in The Force Awakens could have tied her to Luke, who then could have Force projected himself to her in a way similar to how Kylo Ren did. While the relationship would not have been a true dyad, it could have paved the way for Rey to connect with Luke.
Leia Becomes An Even Bigger Part Of The Resistance (& Han Loses Hope)
As for Han and Leia; it's likely the pair would have been in similar, though more extreme, places at the beginning of The Force Awakens. Ben's turn to the dark side was the final nail in the coffin for the pair's relationship, so it stands to reason the devastated couple would still have separated after his death at the hands of Luke. Han, who returned to smuggling after separating from Leia, would probably have sunk to depths lower than he seems to have at the beginning of The Force Awakens. The rough but hopeful scoundrel who puts his life on the line to save his son would have been in an unfortunately much bleaker state.
Leia, who threw herself into the Resistance, would also have probably followed the same path, albeit to a more extreme destination. As someone who fought on through the destruction of Alderaan, the capture of Han Solo, and the turn of her son to the dark side, it's clear that Leia would have doubled down on her war against the First Order. Instead of being the general she was revealed to be, however, Leia easily could have become a more extreme, merciless leader like Saw Gerrera, especially with her own brother as a potential foe.
Would Luke Skywalker Be Redeemed For Killing Ben Solo?
Ultimately, it isn't hard to picture the sequel trilogy having a roughly similar shape, even if Luke had killed Ben Solo. If Luke had done so, he almost surely would have turned to the dark side, and it's reasonable to assume he could have connected with Rey. The most important question, then, would be that of Luke's redemption. Ultimately Luke Skywalker would likely still have found redemption in the end. While many Star Wars: The Rise of Skywalker, then, could have ended with Luke perfectly mirroring the actions of his father and sacrificing himself to save Rey. Whether or not that would have for a better Star Wars sequel trilogy is up for debate.
