wow, is it really so hard for you guys to be serious?
I don't feel that I get scared easily, but I haven't watched a lot of horror movies anyway. Still, I think that a dark atmosphere coupled with some music that makes you feel like there's going to come a zombie and eat your head any second, together with a lot of character animations (aka good graphics) and genious level design could make a game seriously spooky.
Really, I think a good horror game needs all those things up there (including "other"..), since a good horror game isn't something easy to make, especially in a bird-perspective, pixel-art-featuring game maker.
The level design, in my not-so-humble opinion could be so that the player would check around a corner because he seriosuly expects some huge bastard of a brain-eater to come charging as soon as he takes another step, only to realize that the bastard's actually behind him. And then he has to fight the rest of the way through without any brains. Which can be tough, trust me.
Good graphics would mean that it should be more or less profesional, perhaps even making blood splurt out of the zombies you start shooting at. Also, you of course should choose characters to fit the game, and you'd prolly have to forget about using RO sprites for example.
I bet you can find lots of music out there that would work, and if there isn't something made for RMXP, you're sure to still be able to find some BGM or so somewhere. Footsteps for the player and events wouldn't be too stupid either, to make it more realistic.
And realism is what's so important. You have to feel like you're in the game, not sitting by your computer.
The game-play has to be featuring an ABS. Anything else won't work. Ever.
A good title wouldn't be too bad either. Just dont be an idiot and choose something like:
"Sonic and the Spooky Guy Who Forces Him To Go Into Some Maniac Labaratory Which Is Extremely Huge With A Ridiculous Amount Of Enemies Inside With Sick Health That Apart From That Are Like Any Other Enemies That I, The Creator, Have Ever Created" II.
-Eternal