Like some before me I also recommend Vampire The Masquerade: Bloodlines and System Shock.

Perhaps also System Shock's spiritual decendant: Bioshock. But thats an FPS and not an RPG, really.

Technically it's horror as the Kult IP is horror based.

The Thief games are very atmospheric and can get very intense and creepy. Not sure if that suits your style of "horror"; if you wanted more gore, perhaps something like One Unit Whole Blood... Also, while not horror per se, there were many times when playing Morrowind and going into Dwemer ruins I got creeped out, and same for some of the old vaults in Fallout 3. There are also plenty of adventure-genre games like Phantasmogoria or Sanitarium that can freak one out...

That was actually my most disappointing purchase in the recent Steam sale. Looked good in theory, but it's heavily consolised, stutters like crazy when I move around (e.g. pretty much non-stop stutter when pressing two keys at once, such as strafe and forward), weapons break after killing a couple of zombies and they respawn in areas you just cleared two minutes ago.

Not a game I could recommend with all its flaws.
Perhaps, but, either way, it feels like two completely different games stuck together at the 80% point.

I haven't had a problem with stutter, but I will agree about the weapon breaking nad enemy respawns. Still, I think it's solid enough. But some of those mods sound extremely interesting...