This is what luke said when explaining to rey what happened with Ben:

“I saw darkness. I’d sensed it building in him. I’d seen it in moments during his training. But then i looked inside. And it was beyond what i ever imagined ( then you hear lightsabers and screaming). Snoke had already turned his heart. He would bring destruction and pain and death and the end of everything that i loved because of what he would become. And in the briefest moment of pure instinct i thought i could stop it. It passed like a fleeting shadow. And i was left with shame. And with consequence. And the last thing i saw were the eyes of a frightened boy who’s master had failed him.”