Absolutely the scariest thing. Obviously I’m not talking about the immediate threat, but I don’t think there’s much doubt that Kylo brought Luke more pain than Vader (I’m talking emotionally here).

As for Leia’s words, I took it that every Skywalker has Vader in them, Ben (and Luke) included. Yeah, Luke knew he had that potential, but it didn’t scare him enough at first, he thought he could teach him. It wasn’t until he saw it rear its ugly head during moments of Ben’s training that he decided to confront him.