Natsuki Subaru went insane. Using shaula and the books of the dead, he's going around killing everyone that knew the old him to figure out who natsuki Subaru truly is. He wishes to patch together his life, and often talks to people who aren't there. Emilia, Beatrice, meili, Julius. He killed them all. And he plans to extend his massacre. 
He's manipulative and a bit scary, but doesn't kill people who didn't know the old him. He's never died once since leaving the watchtower