If, as a GM, you approach horror by pushing towards it you will misdirect tension, the uncontrolled point will divert. You must pull. Think of horrow like a mystery box at a museum where you must put your hands in a dark slot and guess what is inside. The GM makes the shape inside and hides it, the players titilate themselves by figuring out what it is they’ve got their hands on.
In practical terms this might look like building a simple horrific event and deconstructing it until it is obscured and then allowing the players to put it back together.
I do not believe horror can be organic or player led. I can’t horrify myself any more than I can tickle myself. We are constantly battling against the knowledge that at any point we can walk away from the table and be free.