Yes, it's really happening. Well, kind of.
Al Jean, the show's executive producer, told Entertainment Weekly that Sideshow Bob will finally succeed in his mission, and get Bart next season's Treehouse of Horror episode.
Comparing Bob's chase of Bart to Wile E. Coyote and Road Runner, he claimed "I hated frustration comedy so we’ll scratch that itch".
Jean later stated that it'll just be for the Halloween-based episode, and Bart will return afterwards, so don't get too invested in the storyline. Still the fact that they're planning on offing a 10-year-old on screen is pretty shocking.