The main reason the atmosphere seems so scary to me is just thinking that during the day the pizzaria is supposed to be this amazing fun place that everyone loves and (with the exception of the bite) Freddy and his friends are friendly and absolutely beloved. But during the night, for no real logical reason it becomes this dark horrifying place where you're totally alone and defenseless while you wait for your shift to end. And the fact that the animatronics begin hunting you downright when you show up. Yeah they're supposed to be on some kind of free roam at night I get that, but the way they know when you're watching them and the fact they always go straight to the security room (not to mention Freddy himself actually hiding from the cameras when he's coming after you and foxy only attacking when you're distracted) just makes you feel that there's way more wrong with them than just a malfunction in their programming. And the fact that most of the staff at Freddy's seem totally oblivious to the danger like phone guy just treating it as a mild hazard on the job and even managing to work quite a while before actually facing any real danger, and the owners willing to allow you to work several nights there in the extremely dangerous conditions like there's nothing to worry about. Another thing that freaks me out is that from the way phone guy makes it sound the animatronics just wandered around at night and were only really a threat if you just happen to bump into them, but just a few days before you started there they suddenly attacked and killed him and once you start they actually begin to intelligently stalk you makes it feel like they were actually waiting for you to come there, and were just biding their time until you showed up.