Thanks, folks, who set me straight on IF/THEN. It sounds like post-DC rewrites resolved these confusing issues.
I don't know how I missed this one before, because it comes from "The Phantom of the Opera," my all-time obsession. After Carlotta croaks like a toad during the opera "IL Muto," and the managers promise that Christine will step in in a few minutes, they have the company perform the ballet scene from Act III while Christine is getting into costume. During that time, the Phantom hangs Joseph Buquet. Why would he pick that time to kill Buquet when he knows that he's disrupting Christine's performance? At least the movie shows Buquet and the Phantom chasing each other up on the catwalks, so the Phantom may feel that if he doesn't kill Buquet then, Buquet will kill him.
Both versions follow the death of Buquet with Christine's and Raoul's racing up to the roof and deciding to escape the Phantom by running away. The eavesdropping Phantom sings, "You will curse the day you did not do, all that the Phantom asked of you," after Raoul and Christine go back down. This is where the stage musical gets a bit strange, to say the least -- after Buquet's body is found hanging, and Christine escapes to the roof without her IL Muto costume long enough to sing, "All I ask of you," she returns to finish the performance, after which the angry Phantom crashes the chandelier. This means that, after the body is found hanging, the managers do not stop the opera. At least in the movie, which moves the chandelier crash to the end, there is nothing to indicate that Christine is going back down to perform -- somehow, I doubt that the audience would wait dutifully, while Buquet's body is cut down, the police investigate, Christine disappears to the roof, and then finally changes into costume to finish the performance.
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.