After Robert Bowers stormed the Tree of Life synagogue in Pittsburgh on Oct. 27, far-right personality Ethan Ralph launched a live stream on YouTube to discuss the shooting that claimed 11 lives. Soon, some viewers began paying to have their comments featured on the live chat scrolling alongside the streaming video, through a feature YouTube launched last year called Super Chat.

During the live stream, which YouTube said Mr. Ralph has since deleted, one user paid two British pounds to write, “How u get a Jewish girls number?...