he had a mission to kill you, but he just take you with him to demon realm. he did that to keep you in secret from other people in human realm because you're number one targeted by other kingdom too. you offer him a contract so that he can keep you safe. and after he was save you from the other assassin who's targeted you, he started to like you and his obsession grow bigger. he would killed all people to keep you safe. and that's why he did that, because he know your worth.