I posted that long block of text because it covers the scenes that relate to the current discussions. In TFA we hear from his parents that Kylo was already going down the dark path when he stared training with Luke. He wanted to be on that path. That is a frequent theme with him. He wants to be dark. I think it is safe to say that Luke saw that and knew it was too late.

There is a lot in both movies that reinforces that Kylo is on the path he wants to be. He idolizes Vader. I think this emphasis plus what Luke said is enough to make it a fact that it was too late before Luke ever entered Ben’s hut.

Luke’s hubris and ultimate failure is in thinking he could overcome something like that. His success had gone to his head. For some reason he waited until Ben was old enough to run into problems to start his training. And then Kylo was not only fallen, but he took some of Luke’s other students and killed the rest. How could Luke fail worse than that. All of his students gone in one night. Luke failed in so many ways that don’t seem all that significant until you add them together.