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The basis of horror is the unknown, and there is plenty of unknown -- what is in outer space, including aliens, for example. The creatures in, say, the Alien franchise simply play on the fear of one's body being violated in creative ways, e.g., being made into a host for hatchlings.
Even religious themes play on the unknown, the fear of death, and the fear of suffering. There is malevolence or evil apart from religious ideation. I daresay that The Exorcist or Rosemary's Baby is as scary to an atheist as to a theist, because the horror is based on the distortion and brutalization of humans. It just so happens that the backstory for the entities doing said desecrations is religious. It needn't be.
It was a good time for me, the only downside was knowing that I'd wasted so much time and mind on this poison called Christianity.