Multiplayer can't be scary.
Actually, it can be, so long as it's done right. And, I think a lack of weapons really helps. The other thing that would help would be going out of your way to assemble a group of four close friends to play with you rather than random ten-year-olds.
A situation in which your only allies are killed off one by one by an unstoppable monster that stalks your every move is just as frightening as facing the same situation alone. That's how multiplayer can be scary.
Multiplayer KILLED the System Shock 2 experience for the sole reason that you could fight back against the monsters, and now had twice the power. Even if the game was locked into insane difficulty, it was easy as pie, and much less scary. The same thing can't happen in a game where the only enemy is an invincible one, especially if he can one hit kill you. It will always be scary... Unless you're in a server with an annoying kid running around shouting about how he banged your mom last night. Two things are certain in that situation. 1.) He will die first, and 2.) there will be much rejoicing at his demise.
