This post is a response to @The Smiler Human
If that is a mouthful idk what mine is gonna be, but anyways
Mindless Creatures
Sure, insects are creepy and gross enough based on appearance alone, but what truly makes them scary for me is how they call into question the intelligence that we as humans value so much. Swarms of relatively mindless insects can pose an existential threat to us, all without needing the ability to speak or think introspectively. They make us question how important intelligence is in the grand scheme of things. After all, how valuable can it be if creatures like worms and cockroaches will probably end up outlasting humanity?
Human Cruelty
Even scarier still would be the capacity for humans to be cruel to each other. I especially realized this when editing the Wormlings page, as no matter how horrible Wormlings may be, the thing that truly makes them horrifying is how some humans are willing to exploit them and other humans for money, going as far as to feed living humans to the creatures in order to make more of the drug. It's kinda like drugs in the real world, I think whats really scary about them is the violent criminal organizations that profit from the drugs.
Demonic Beings
An old school fear, but still frightening nonetheless. The thought that some creatures might exist solely to torture and torment humans is pretty darn horrifying. They are like H.P. Lovecrafts monsters if they actually CARED about hurt humans; in fact hurting humans seems to be the ONLY thing they care about.
And demons aren't usually just mindless brutes. They are definitely brutes, don't get me wrong, but they have the mind of an eldritch god, and know things which could probably make you go mad, or probably know more about human nature than you ever will. They can choose to tear you apart limb from limb, or fill your mind with disturbing images to make you go insane. It's all in a days work for a demon.
Isolation
Sometimes, the absence of other beings can be scarier than the presence of monsters. It's good to be alone every once in a while, but I don't think anyone truly wants to be alone forever, which is why the Backrooms itself is so innately terrifying. Sure, there are groups and colonies scattered about, but in this infinite expanse, you can go a lifetime without encountering another wanderer at all, depending on where exactly you are in the Backrooms. The first time I was truly frightened by the concept of loneliness was the first time I watched Spongebob, ironically enough.
This episode messed me up as a kid and awakened in me a deep existential horror that pushed me to learn more in order to combat it this fear with understanding.
The Power of Nature
There are so many examples of the frightening power of nature, and this may be one of the most primal fears of all. Just simply the world around you being hostile and able to be sent into upheaval in mere moments, leaving you utterly powerless before its awesome might. The nature around us that we usually think of as inert and static is not the Backrooms. It is not artificial or contained indoors, but instead wild and with a life all it's own that consists of destruction and creation, a cycle that will go on and on long after we are all dead.