“Don’t Let Them In” (*1/2 out of four) was a joyless horror show about two social-service agents (Michelle Luther and Aidan O’Neill) who follow an investigation to an abandoned hotel where they are suddenly besieged by masked intruders who are trying to kill everyone. Cheap production values and ugly photography negate both Luther and O’Neill’s good performances and camaraderie together. Both “Vacancy” and “The Purge” told similar stories with far more style and scares. “Don’t Watch” would be a more apt title for this timewaster.