Buffy the Vampire Slayer” was set at a California high school beset by vampires, demons and toxic mean girls, but it turns out its real-life big bad was the show’s creator, Joss Whedon.Then in his early 30s, Whedon — who built “Buffy” into one of the most beloved and influential shows of the past 25 years and went on to write and direct 2012’s “The Avengers,” which launched the biggest movie franchise of all time — was heralded for his witty dialogue and wrenching plot twists.