Complex sentence: When Will sees a shape approaching from the far end of the hall, he takes Laurie's handgun and shoots the shape five times, only to discover that it was the school's security guard, Ronny (LL Cool J).
Split into simpler sentences: When Will sees a shape approaching from the far end of the hall, he takes Laurie's handgun and shoots at the shape five times. When they check the downed shape, they discover that it was actually the school's security guard, Ronny (LL Cool J).