In the Star Wars sequel trilogy, Kylo Ren -- also known as Ben Solo -- believed he needed to kill his parents in order to remain on the dark side. While he had mixed feelings about Han Solo and Leia Organa, his repeated hesitation and conflict demonstrated he still cared for them. However, Kylo felt a deep hatred for Luke Skywalker, his uncle and former teacher, Luke Skywalker. While much of that originated with Luke's attempt to kill him, Kylo always had a heavy distrain for his one-time mentor that stretched to the very beginning of his training.
After Han and Leia reunite in Star Wars: The Force Awakens, they discuss the mistakes they made with raising their son, Ben. Han believed "There's too much Vader in him," to which Leia replied, "That's why I wanted him to train with Luke. I just never should have sent him away. That's when I lost him." That not only set Ben on the path to the dark side, but it also began his hatred of Luke.
Kylo's Hatred Began With His Jedi Training
Although Ben was a problem child, there remained plenty of good in him. But much like his grandfather, Anakin, he was sent away from his parents to be trained as a Jedi (in Ben's case, with Luke at his Jedi Academy). That was, easily, the biggest mistake. Not only did Ben care little for the Jedi, he also viewed Luke as the person keeping him away from his parents. And the strict rules of the Order only made matters worse, as Ben felt they limited his potential, and forced him to forgo attachments.
As revealed in the canon comic series Star Wars: The Rise of Kylo Ren, Snoke began tp appear to Ben as soon as he started his Jedi training. Through countless lies, Ben was convinced the Empire helped the galaxy, and that its downfall was all on Luke. Although Luke was celebrated as a hero of the Rebellion, Ben saw him as the Emperor's murderer, and someone who tricked Darth Vader into becoming weak.
Over time, Ben believed he could become as powerful as his grandfather, Darth Vader, once was. But with that level of power being frowned upon in the Jedi Order, he assumed Luke was purposefully holding him back. While Ben's training has barely been shown, it's safe to say that he and Luke didn't get along. That disdain quickly turned to hatred. It all came to a head with Luke's momentary lapse of judgment, where Ben witnessed his master hold a lightsaber above him. With that, all the lies about Luke being evil were seemingly confirmed.
Kylo Finally Confronted Luke in The Last Jedi
By the time of The Force Awakens, Kylo's hatred had only grown stronger. He was actively searching for Luke in order to kill him, despite the self-exiled Jedi posing no threat. Knowing that Rey, the only person to whom Kylo felt a connection, was now training with Luke only made matters worse. Everything culminated in their duel in Star Wars: The Last Jedi, where Kylo ordered, "Every gun to fire on that man!" Yet, Luke still left on top, as it was all an illusion to distract Kylo from the escaping Resistance.
In the end, Kylo lived a tragic life, and much of his frustrations were pinned on Luke. While the former master was at fault for some of his actions, the main issue came from Snoke plaguing Ben's mind. And, if Ben had been left at home, with his parents, he likely wouldn't have followed his dark path. Leia was the only one who could bring him back to the light.