[yotsuya said:]
Ah, but Luke never says what Kylo would do, only that he was already full of darkness and would do bad things. What he saw he does not elaborate on. For the purposes of what you are saying, we can assume he did not see the galaxy wide distress Kylo would cause.
Luke literally says, I saw the darkness and the suffering it would cause. He saw what Ben would become. He then lit his lightsaber in order to put a stop to it. Considering what he had seen in his youth, it must have been pretty terrible for him to freak out like that. So, yes we can assume he saw the galaxy wide distress Kylo would cause.