I’d prefer a consistently frantic mood. Always taking care to look behind your back for a threat that could come at any time. A quiet ambience and a somber soundtrack would play a key role here imo as well as aesthetics. Then there’s when you’d be chased. Cramped spaces, some trifle debris on the floor, dim lighting together with rapid background music would all contribute to a sense of claustrophobia and heightened tension.
On the matter of the map’s environment, I notice most maps make heavy use of fog and dark skyboxes/lighting and a uncanny soundscape which doesn’t always have to be the case. If you can do so effectively, you could have a setting in a seemingly safe environment only to later reveal how dangerous it really is. The only examples I can pull out of my mind right now are from creepypastas.
Music isn’t always integral; You should only use it moderately, at places where it is most appropriate. Too much music can be fatiguing to the ear imo.
If you need some inspiration, the most obvious source would be to read some horror fiction and watch horror films/videos. There’s this thread in GD with a ton of really fucking creepy shit including videos, pictures and even solely audio. Also, you can try visiting the Creepypasta Wiki. It’s loaded with a lot of scary stories though most of them aren’t exactly good.