[ He slams his fist into the ground upon hearing Hux's explanation of Luke's final moments. He knows the man did it simply to fuck with him for one last time, knowing he won and Kylo lost. It was a final fuck you for going to the dark side. Luke, unlike his mother and father, had completely given up on Kylo. He knew what Kylo had become, knew that he was no longer Ben Solo, and was content in letting him continue down that path.

So... he left, becoming one with the Force. And Kylo is incensed.

He knows Luke wasn't even there standing against him on Crait, not if he just disappeared like that, leaving behind absolutely nothing. ]

I didn't kill him, then! Don't you see that? Even a Jedi will have a reaction to lightsaber blade going through them, but you say nothing about him falling to the ground or grabbing his midsection, nothing.

[ He wishes he could see exactly what happened for himself, but he knows there's no way to do that without forcing his way into Hux's mind... And, even then, he's not sure he'd see what he needs to. Everything would be from Hux's point of view. ]

If he just vanished and left nothing behind, he wasn't even on Crait.