This sounds like a great idea however...
Seeing your screenshots worries me. You have no hud, making the headshot very difficult to pull off, and it is very dark. One succesful attack by a zombie results in your death. Think about wandering through a nearly pitch black room with no targeting reticule and having a zed jump you. It will be difficult to succesfully get the headshot off, especially in dark, cramped quarters. I've never played the game, so I wouldn't know how it's gonna work.
My suggestions:
1: Make the earlier levels daylight. It can't stay night forever. Keep it mostly light and then throw a tunnel or nighttime level in to throw the player off.
2: Keep the no hud thing, I like that

3: To enhance the horror, now that it's light out, give the player absolutely no instructions. All the player needs to know is the storyline, and maybe that a bite will kill. Let them figure out the rest themselves.
4: Use the environment. Throw objects at their heads, barricades, etc.
5: Instead of instant death, give the player a health bar he cannot see. If possible, although this may be tricky, give the zeds a script similar to rolfy's poison spiders. The outbreak could be caused by a bacteria or parasite, thus allowing you to save yourself by finding antibiotics in a medkit or alcohol.
These are just suggestions, and like I said I probably don't know what I'm talking about. I really hope this game turns out well. Good luck on it.
-yoad
Avoid eye contact...
...if no eyes, avoid all contact.