The murder caused by Murray’s murder occurred when Thomas, Martha and Bruce were leaving a movie theater showing Zorro, Gay Blade. And any Batman fan knows what’s next: Waynes was shot and Bruce watched them slowly bleed. From there, he took his first steps to become a Dark Knight.
For everyone, though not directly doing but the whole thing was sparked by Arthur Fleck. In this universe, Joker created Batman. When asked about the joke that made him laugh at the hospital staff, the first thing that made Joker’s mind flashed was that Bruce’s parents were dead.