Maybe some of the people on board had a laser gun when they got infected, and died fighting, so the zombies have guns and shoot at you. This would be interesting, because if you can make enemies that shoot, maybe one of the bosses could be a guy with a gun who joined forces with the zombies to kill you or something. Wasn't there going to be a giant mole boss that went underneath the floor and came out under you and attacked you, and you could only shoot him when he was above ground, or am I crazy? Anyway, if you had enemies with guns, you could change the sprites, and poof, it's a war game.