Here are my suggestions of making a horror map:
-Lighting almost makes a horror map, you wouldn't want a map almost full bright, and see everything that comes out at your, dim the lights a little, maybe make some lights flickering etc.
-Make broken architecture, objects etc. dont make a place clean and tidy, and say it's spooky, make it look like people were getting attacked, put stains on the walls, blood decals on the wallsfloor etc.
-Make some backtracking, like if the player runs through a horde of zombie monsters, and gets to a locked door that says "Key Required", then the player has too get by the monsters again to go back and look for the key.
-Cutscenes at the beginning of a horror map wont hurt, like make the cutscene show some zombies eating people, walking down a hall, breaking a door, just to tell the player when he starts the map, that he's going to be scared.
They're just some suggestions.