When I say that horror is not a genre, but a mood, a suspicion, a perspective, I assert this not to condemn genre, but to recognize that the qualities I love in one type of story can often be found in unexpected form elsewhere.
To give one example: here is a passage from an Isaac Babel story, "Crossing into Poland."
"The commander of the VI division reported: Novograd-Volynsk was taken at dawn today. The Staff had left Krapivno, and our baggage train was spread out in a noisy rearguard over the highroad from Brest to Warsaw built by Nicholas I upon the bones of peasants.
"Fields flowered around us, crimson with poppies; a noon-tide breeze played in the yellowing rye; on the horizon virginal buckwheat rose like the wall of a distant monastery. The Volyn's peaceful stream moved away from us in sinuous curves and was lost in the pearly haze of the birch groves; crawling between flowery slopes, it wound weary arms through a wilderness of hops. The orange sun rolled down the sky like a lopped-off head, and mild light glowed from the cloud gorges. The standards of the sunset flew above our heads. Into the cool of evening dripped the smell of yesterday's blood, of slaughtered horses. The blackened Zbruch roared, twisting itself into foamy knots at the falls. The bridges were down, and we waded across the river. On the waves rested a majestic moon. The horses were in to the cruppers, and the noisy torrent gurgled among hundreds of horses' legs. Somebody sank, loudly defaming the Mother of God. The river was dotted with the square black patches of the wagons, and was full of confused sounds, of whistling and singing, that rose above the gleaming hollows, the serpentine trails of the moon."
[From The Collected Stories, edited and translated by Walter Morison. Meridian Books, 1960.]
Everything I love in horror fiction can be found here: similes that convey unease ("like a lopped-off head"), vividly-evoked settings full of troubling details ("the smell of yesterday's blood, of slaughtered horses"), metaphors that suggest an uncanny threat or process ("the serpentine trails of the moon").
But do these details (and grimmer ones to come) make this a horror story?
One response would be to say, "Horrific matter, horrific moods, make a story horror." I can understand this, and I'm tempted to agree with it. But I'm also tempted to say that what we normally consider horror fiction is just one small group of stories within a larger framework that acknowledges the power and presence of a mood. Horror is an inescapable aspect of life; it is not limited to a genre.
How you feel about this, and whether you agree or disagree, will depend on your own perspective, and I wouldn't have it any other way. But from my point of view, this ambiguity, this uncertainty, implies a freedom to explore the nuances of a mood without limitations of style or content or expectation. Not only is the road wide open, but there are many roads, and they all veer off towards a storm-cloud horizon without fences and without maps.