From the first minutes of “Murder House” through the latest episode of “Freak Show,” “American Horror Story” is responsible for countless nightmares thanks to its weekly doses of disturbing moments. Co-creators Ryan Murphy and Brad Falchuk (and their writers) consistently outdo themselves when it comes to grisly killings, brutal sex scenes, and creepy characters, campily intermingling supernatural tropes with terrors that are grounded in reality.