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what do we fear most about the undead?
I think it is that they are malicious - they are consumed by a purpose to bring harm to you from which they cannot and will not be swayed. They are enemies who seek to hurt you in some significant capacity and they will not abate from this attitude no matter what happens, unless they are stopped, it is only a matter of time before they succeed or die trying, if they arent trying to harm you they will be after someone else instead, or they will be unaware that harming you is an option on their table at the momment.
a smart zombie is a human being that wants to cause you some significant kind of harm.
atop of this we add details that make the situation worse, such as modifications to their appearances, natures, causes, or even motives.
what of they are apologetic attackers? they have to kill you for some reason, such as for food, even though they dont really want to.
perhaps they are the type that can look and act just like us (in order to prey upon us better), and so there is the element of paranoia present amongst the survivors.
they could be the product of infection (resident evil), but also of mind control (condemned criminal origins), or demonic possession (until dawn).
they could be living beings, or dead.
as a result of their states, they could be as capable as a human being, or less, or more.