Hi. I have some ideas for you, although you are already half way done, they might not be as helpful but I will still tell them to you.
I didn't think of this idea my self (at least the build up and the environment part), it was from another forum. The environment is very important to a horror campaign. It doesn't have to be at night, but foggy is a must. The first two to three missions should be a build up for the horror. No zombies yet. The soldiers, or who ever, find abandoned towns and things like that. Then after those build up missions, you go to a town and you *think* it is abandoned to. You are about to leave and the driver is having engine trouble, you wait around then notice people walking to you in the fog. The troops yell at them like, "come over here!", "where have you all been", and such. And when they come close enough the troops notice they are bloody. The troops don't know what to think. They notice they are surrounded. The troops warn them to leave, and then open fire. You have to battle them off until the driver fixes the engine and you can drive out of there. After that, is up too you.
Another idea is maybe another country hears about the disaster. The get units to an off area air base (like in Nogovo's north airport... it isn't connected to the island)-- this would be latter in the campaign. The know zombies are involved, but other then that are just rumors. A few heli flies to help the troops on the mainland. You second character's (he would be a combat medic) heli happens to land in a bad spot. Zombies are surrounding you and bit a few teammates. Since the Intel about the zombies is quite low, you ask for a medic heli to pick up your bitten comrades. It does and you fly back to the main base. You put the troops in a tent with bitten troops already in there. You walk the troops in and lay them down. You are trying to help them, then all the sudden your first character walks in. he yells at you and says "they are going to become zombies, kill them before it's too late!" but it is too late, they become zombies. You two are stuck in the middle of them all. If any of you die, it is mission failed. You have to get out of there and then get a fellow tank to blow them up once you are out.
I think at the end, every on should die. It would give it a nice dark zombie movie feel. But before the campaign is over, there should be something that you think would be hope, like a type of vaccine or something, but like in Day of the Dead, it all goes to heck. And instead of your characters living, I think no on should live... but it is up to you. Hope I helped.