New narrative clues found in the latest trailer for Star Wars: The Last Jedi imply it was Luke Skywalker who pushed Ben Solo to the dark side. When the son of Han Solo and Princess Leia became Kylo Ren, Luke felt responsible for what transpired and went into exile on the planet Ahch-To. There, he was far removed from the conflict as he searched for the first Jedi temple, but eventually a shot at redemption presented itself. At the end of The Force Awakens, young scavenger Rey ascended the Jedi Steps and found a weary, grizzled Luke at the top of the mountain, offering the legend's old lightsaber with her outstretched hand. Many viewers had presumed in The Last Jedi Luke would train Rey, much like Yoda had taught him.
The Episode VIII theatrical preview that was released earlier this month made it seem like things will not play out this way. While Luke initially agrees to show Rey the ways of the Force, he soon becomes frightened by the immense power she displays and turns his back on her. One of the most fascinating bits of dialogue in the trailer is Luke saying to (presumably) Rey, "I've seen this raw strength only once before. It didn't scare me enough then. It does now."
This statement reads as a clear allusion to Ben, who was once Luke's prized pupil before being seduced by Supreme Leader Snoke. While it's all been confirmed Kylo Ren was the one who ruined Skywalker's new Jedi Academy, his reasoning for turning has been unknown to this point. It remains to be seen if the sequel trilogy (or another piece of canon) will answer the question of why Ben went dark, we have a theory that it was Luke who sent him down that particular path.
What We Know About Ben Solo
Before we get to Luke, it's worth recapping what's been revealed about Ben Solo in the new films so far. In contrast to several of the Star Wars villains we've seen onscreen over the years, Kylo Ren has not yet fully committed himself to the dark side in The Force Awakens. He spends a majority of the film torn between the light and the dark, looking for the proper guidance from Snoke and even Darth Vader's melted helmet. As J.J. Abrams and Rian Johnson have said, there's still molding to be done in regards to Kylo's arc in the trilogy, making him one of the most interesting and nuanced characters Star Wars has seen. Killing Han Solo only made his turmoil worse, and now he's more troubled than he was before.
At San Diego Comic-Con 2017, legal professionals made the case that Kylo Ren suffers from a crippling addiction to the Force (perhaps making him not guilty of murder) that negatively impacts his ability to form a mental state and act within his own free will. His obsession with the dark side controlled his actions, making him do things he feels like he has to do as opposed to what he wants to do. This is very apparent as Kylo confronts Han at Starkiller base, where the youngster is begging his father for help, trying to convince himself he has reached the point of no return. And since shortly before The Force Awakens, Ben was still learning under Luke's wing (see: the novel Bloodline), he's only recently gone to the dark side and has to prove himself a worthy apprentice to Snoke. Destroying Luke's academy was probably his first "test," followed by his fateful meeting with Han.
Based on footage from the Last Jedi trailer, Kylo is still struggling with his allegiances when Episode VIII starts up. In the space battle, he is on the verge of tears as he targets General Leia Organa's flagship, and his destruction of his helmet is a symbolic gesture - indicating he's having doubts about the dark side and is ready to become his own man. When Anakin Skywalker turned, he didn't blink at killing younglings, slaying the surviving Separatists on Mustafar, and jumping into battle against his old friend, Obi-Wan Kenobi. Kylo, on the other hand, still seems he's unsure if this is what's best. It appears as if he's merely giving the dark side a trial run instead of wholly pledging himself to Snoke's teachings. The Supreme Leader is even a little skeptical of Kylo, feeling Ben Solo's emotions made him weak.
Despite dressing in all-black attire and wielding a red lightsaber, Kylo is not yet completely persuaded by the dark side, seeing if he's cut out for their ways only after losing faith in the Jedi. Since Ben didn't have a wife he was trying to save from death or was blatantly ignored/neglected by a self-important Council (that we know of), the one person that probably made Kylo's dedication in the light side waver was the same one that was supposed to steer him straight: Luke Skywalker.