nice images here
I think you have what you are trying to portray down pretty well but alot of the pieces could do with some more contrast.
For example, i did a quick paintover of the zombies its literally just an overlay/soft light layer with some darker and lighter colours on it to really bring out the awesome detail you put into some of the characters. That said less is sometimes more and its alot more scary when you cant quite see everything - like just pairs of eyes in the background hinting at unknown numbers I think is scarier than seeing every zombie.
Almost all the images suffer a little from too much ambient lighting, there is no contrast in them and when you are trying to create tension knowing where and how to use light can really help. Roughly the lightest part of your picture should be the focal point and create a starting point for your eye to then travel around the image and end up back at the focal point.
