By Michael O’Sullivan, The Washington Post M. Night Shyamalan just can’t leave well enough alone. In “Split,” the writer-director’s new psychological thriller, “well enough” is James McAvoy’s remarkable (if distractingly showboat-y) turn as the film’s villain Kevin, a man with dissociative identity disorder (previously known as multiple personality disorder). Kevin’s 23 distinct alters include the effeminate fashionista Barry; the creepy neat freak Dennis; the matronly den mother Patricia; and an inquisitive, hip-hop-loving 9-year-old with a lisp named Hedwig.