Between the Dark influence he felt hints of in his brief encounters with Ben Solo and the Force showing him a hint of the future, Luke didn't just know the massacre at the Jedi school would happen, he knew about it years beforehand.

Even though he knew he could probably stop it from happening with that much warning, he knew the consequences would be far worse. But he wouldn't let that stop him from sending his daughter away to Jakku as soon as he knew, before Ben Solo even arrived at the school. He has been tormented by what he had to let happen ever since.


TLDR: Luke knew about the coming massacre years in advance and did nothing but send his own child out of the line of fire.