In Star Wars: The Last Jedi, after Luke has witnessed how powerful Rey's ability to use the force is, he says to her:
I've seen this raw strength only once before. It didn't scare me enough then. It does now.
When Luke talks about seeing that raw strength only once before, presumably he is referring to Kylo Ren. But if that's the case, then saying "it didn't scare me enough then" doesn't make sense. Luke saw Kylo Ren's strength and it scared him enough to consider killing him in his sleep. So what did Luke mean exactly?