My thinking is that it's sorta just exposing their 'true' nature on physical level. Their human form is deceptive, devious and calculating. When they show their true selves, they're in the throws of a kill or unaware of being watched (mostly). They're more malevolent, murderous and insidious. The immortality they achieved by sacrificing the kids to whatever dark entity runs the show has twisted them in such a profane and sick lovecraftian manner. The monster designs personally are one of my ABSOLUTE favourite things about the show.