Spike held in his claws the key to the Door of Temporas. With it he could travel back to any point in the past. He could change it all. She didn't have to die. However, Twilight Sparkle had sworn an oath as leader of the Temporal Control Agency that she won't allow alterations to the past. Any change to history could result in millions of ponies never being born, the fates of entire nations could be changed, the world could be destroyed for lack of a single tiny, but key, event. She had to protect the timeline, even if it meant breaking one friend's heart and leaving the other dead and buried in the past.