Luke intends to talk to Ben. He knows he's been putting it off for too long, watching the presence of the dark side's pull grow in his nephew while not trusting himself to try and do something to stop it, too afraid to possibly influence him in the wrong direction. He can't wait any longer.
When the knock on the door of the hut where Ben resides returns no answer, he expects his apprentice is out somewhere and lets himself in, thinking vaguely to wait for him to return. And yet there the boy is, in bed, just a tuft of dark hair emerging from the blanket. Luke comes to stand beside him, the aura in the room one of simply peace and rest.
And yet when he holds out a hand, concentrates, the deeper machinations of Ben's mind sink into his. Evil. Destruction. Death. Luke catches his breath sharply at how far the corruption has already gone. The visions he sees of the ruin Ben will bring– He knows he's too late.
Luke's lightsaber is in his hand in a second, blue blade too loud and bright in the dark room. Instinct commands him, the weapon slicing elegantly, effortlessly through the air.
And he seems to be waking a second later, the smell of singed fabric and burned flesh assaulting his nose. Luke can barely process what he's done as the lightsaber clatters to the floor, deactivating with a hiss.
On the bed, Ben is- his nephew is-
The feel of death slithers through the Force, and for long, long moments, Luke can do nothing. Not breathe, not move, not think.
He's done this. He's killed Ben. There's no taking it back, as much as the horror claws at his throat and makes him feel about to be violently ill. Luke finally breaks his trance and jerks away, heaving powerfully, though nothing comes up.
He stumbles from the building, leaving the lightsaber, the carnage and evil he's wrought, behind. Outside, the air is still and quiet, but it bleeds. Anyone would be able to sense it, even his weakest Padawans.
Luke runs a shaking hand over his face, feels tears there. What does he do now? What does he do? He thinks of Han, of Leia. They'll never forgive him. He wouldn't expect them to, knows he will never forgive himself. He of all people knows someone gone to the dark side can return. There was a chance for Ben. There's always a chance, and now, this great act of evil Luke has committed...
He runs, drawing on the Force to move quickly and silently across the Temple grounds, straight to the place he keeps his X-wing. He doesn't pack. He doesn't plan. He just has to leave.
It occurs to him as he starts preflight on the old ship, already missing R2 but not willing to drag the ever-loyal little droid into this, that he could lie. No one would ever suspect him of this. Yet even now, the dark drawing close around him like a pack of predators sensing blood, he can't do that. Han and Leia deserve better. His students – former students – deserve better.
He takes the ship off the ground and shoots from the atmosphere, knowing somewhere below someone is probably watching. There will be questions. An investigation. Even if they don't suspect, they will look for him. He also knows their search will be fruitless.
The galaxy spins, and balance shifts. The potential darkness in Ben Solo is snuffed out. The light of Luke Skywalker is dimmed. The Force moves, already working to course-correct. What is meant to be will yet find a way.