The appearance of the animatronics in unexpected places is really scary. For example, when they suddenly show up in the dark corners of the pizzeria in the stories.
The mystery surrounding the backstories of the animatronics is quite scary. You don't know what really happened to them before they became these haunted - like beings. It makes you wonder what kind of dark secrets are hidden within their mechanical frames. And as you read through the stories, the sense of dread builds up as you learn more about their possible motives for scaring the characters in the stories.
In my opinion, it's when the power goes out in the pizzeria. Without power, the security systems fail, and the animatronics can move freely. The main character is left in the dark, not knowing where the animatronics are or when they will attack. It creates a sense of total helplessness. For example, in a story where a guard is trapped, he can't see anything, and all he can hear are the creaking and groaning of the approaching animatronics. It's a moment of pure terror.
The animatronics' menacing appearances are quite scary. Their distorted forms and the way they move in the graphic novel can send shivers down your spine.
Well, there's a version where the pellets that Pac Man eats are actually souls. That's really spooky. So every time he munches one, he's consuming a part of someone's essence. And the ghosts are the protectors of these souls, trying to stop Pac Man from his evil deeds. This version really changes the whole perception of the game and is quite terrifying.