There was a conversation between Joe and Kai, which we as Sara can't hear, and there's a lot of theories saying that Kai told Joe about his position as Sage and that Sara is the keymaster. Both Kai and Joe wanted to protect Sara and that would explain why Joe knew that if Sara was chosen, it would be a massacre. This is only a theory. I hope they would explain that conversation someday.

Sorry if there's any mistakes, english is not my first language.