Complex sentence: When she discovers that he is destined to become a Demon Lord, she decides that it is in the best interest of society to destroy him, believing that he knew about his destiny and that he tricked her into becoming friends.
Split into simpler sentences: When she discovers that he is destined to become a Demon Lord, she decides that it is in the best interest of society to destroy him. Initially, she believes that he knew about his destiny and that he tricked her into becoming friends.