Okay, I thought I'd make a "guide" of how to make a scary game.
I know there's been many already by many people, but... I thought I'd do one too.
So, here goes:
-Ambience. You should have dark places, with not too much scary things, just equally "scary" and peaceful. You know, like, maybe have some scary ambient sounds all over, but don't have zombies, monsters, abominations, gore, etc. everywhere, because it's easily getting boring and not scary. Just place a few enemies somewhere in dark corners, behind windows, etc, so they scare the player.
-Don't give the player a weapon in the start. And don't make the weapons to be found too obivously, and not much ammo. Never have enough ammo to kill everything. Running away from zombies is scarier than shooting everything. Though it's a matter of taste and stuff.
-Good graphics. Well, I'm not saying this 100% seriously, as we all know FPSC's limits, but I mean, menu, huds, etc. also.
For example, don't use huds like this:

Because well, for OBIVOUS reasons, that does'nt fit in a horror game.
But, use huds like this for example:

Note that there's no numbers, because that makes the player more careful when playing the game, as he can't know how much health he has.
-Use "obscure" things, for example, have some things that can be seen, but they're not obivous. I mean, like subliminal scares. Well, that's really hard to pull out well, but if you succeed, you'll succeed making a scary game.
-Don't put lights everywhere. Have some dark places where the player must go through, and before those, make something scary come from the dark place. I'm sure 50% would pee their pants and stop playing horror games for the rest of their lives. (or maybe not)
Here's an example screenshot of a thing that probably no one would even notice, but if they do, they're like "wtf is that... is it a handicapped human or a deformed dog!?"
-Use cutscenes. Well, not too often, but for example if you have a tv, make a small cutscene (like half a second or less) so it flickers on with some really dark in it, something that is almost not noticeable, but the player notices the game doesn't lag, but that something happened, so it can still haunt the player.
-Use sounds. There's the "traditional" way to use scary sounds, scary music, etc. but if you'll use something not intended to be scary, it's a lot scarier most times.
-Don't use sounds or jumping things at the player's face. I mean, have a really dark place where the player can barely see anything, and so there's no sound at all, except footsteps maybe. And then have some clues for the player so he should for example open a door or window or something, and keep the tension really high, so they KNOW something horrible is going to jump straight to their face, and then DON'T have anything happen. They'll pee their pants and stop playing, crying for mommy.

(well, not all, those who're scared of only things jumping in their faces won't be scared, but well...)
Here's examples of dark places where the player just HAS to go, even though they know there's happened something unpleasant just a moment ago...

Just ignore the pink light.
Now would you like to go there?
-Things jumping at the player's face. Okay, I just said DON'T have those, but of course, they are scary too. So, make an enemy spawn behind a window when the player walks there, he gets some nice new brown pants.

Or just behind a corner where the player turns, unexpecting anything, and then, BAM! A guy with a nice little gun starts shooting them.

Well, usually humans as enemies is not scary, so replace that with a crawling zombie,

and the player wakes the whole house when he's playing in the middle of the night.
-Narrow edges, which the player might easily fall off, and have him be careful not to fall, and then something scary happens, and he'll drop from the bridge or whatever walking on, in the middle of a swarming pit of zombies.

...Or lava, or whatever suits you best.

Not the best example, but well, you get the idea?
-"Twisted places". Well, I'm not good describing, so I just show an image to demonstrate:

Not really scary, but it might be scarier than a normal place.
-Crazy npcs. Well, it's always scary if there's some mad old guy who talks something creepy like "Mommy... Daddy... Help me, please..." or something. Or just weirdly placed people, so they stare at walls, or something.
Here's an example:

Well, this scene isn't so scary, but if that non-scary guy would be replaced with a skinny, naked bald man or a baby, and the "scifi-like" place would be an asylum or something, it'd be scary.
Okay, I saved the best for the last. Well, the unexpected one in this case...
*drumroll*
THE BLUE SCREEN OF DEATH!!!
Okay, maybe not scary, you think, but imagine a room FULL of those. Still not scary? Well, then, think about it if you saw it somewhere REALLY happening? Wouldn't it be scary?
Well, yeah. I consider it scary. You don't, okay, that's fine. I've always been a little weird.
EDIT:
I said I might update this... so here goes.
-Things behind doors.
Well, this is common sense, but well. I'll put it here anyway.
So, a door, and then behind it... something scary.

For example, if you'd see a door like this in a very dark place...
You could expect something scary behind it, right? So, make some scary sound come right when the player opens the door and...

Well, I used a body for this, not the scariest thing, but if you'd put a horde of zombies there, jumping at the player... That'd be scary as hell.
-Hellish places. Weeeell, kinda common sense too, but still, here I'm writing about it.

So, well, anyway. Hellish environments can be pretty unpleasant...
Again, not the best example, but you know, hellish like.

So, well... If you'd enter these rooms irl, you probably wouldn't drop some homemade brownies in your pants, but it could be a little scary, I think.
.........
I might update this sometime with more things. But for now, enjoy this little guide.
Anyway, now, PLEASE COMMENT!!! Is it useful or not? Should I continue writing more about these things?