"Nightcrawler" isn't a horror movie, but it might be the most frightening film at the theater this weekend. In his directorial debut, screenwriter Dan Gilroy ("The Bourne Legacy") points his lens at a subculture that's usually behind the camera: the freelance videographers who prowl the inky, glittering city streets at night, tuned in to police scanners, waiting for some kind of tragedy to break ? a car crash, a fire, a shooting, whatever. The bloodier the better. If they're lucky, their footage?