If I'm on a platform or building, I can aim downward, but not quite enough to shoot a zed... Ok, so the zombie will move when I move so that it is directly below me. If I aim down, I am limited to aiming a few degrees from straight down, like an imaginary cone that I cant shoot into, and I can't aim at the zombie. If I move to aim, so does the zombie, and now I have the same problem again. In real life, I can, if I really had to, aim down directly between my feet (not that I would ever want to do that :-P )