In one of the Worlds I am building, there are zombies. Sadly the zombie genre is dead(sorry), this is mainly due to the overwhelming domination in the genre and lack of new creative zombie types (world war z, 28 days later, etc). To solve this I have simply made my zombies as smart and fast as humans, but this creates a problem. Why wouldn't the infected use any kind of weapons. Why How do I explain why the zombie don't use tools or guns?
Perhaps the virus/bacteria/fungus/nanobot/whateverelse affect the cerebrum. According to this article on the anatomy of the brain, the Cerebrum is responsible for the following;
It could also impact other areas of the brain in varying ways. For example, if you hit the brain stem and cerebrum, you can literally drive someone mad, especially when combined with the loss of emotions and judgement. For story purposes, you can have your virus target only parts of the brain that relate to specific behavior. The cerebrum can cause violent spasms if damages.
Anyway, the reason I mention the cerebrum is that if you take away a person's coordination as well as parts of the cerebrum you can have a hysterical, not-so-stupid zombie with lack of coordination and spasms. When combing these two with guns, you would get a self-destructive combo. Any and every self-respecting zombie would quickly realize that guns = death. This would not stop them from using crowbars and close-combat weapons, but it will ensure they won't overrun the police station and go on a shooting-spree GTA style.
Otherwise, I do not think what you ask for is possible. You ask for intelligence, but no tools, which is essentially what weapons are; the only viable tool to a cannibalistic madman/madwoman. You can biologically limit the weapons they can use, but if they have even the intelligence of a monkey they will eventually pick up a crowbar and learn that swinging it hard enough can help them kill prey.
EDIT: I misread the question as being how do I describe a zombie that uses weapons... damn lisdexia. Anywho the approach would be the same. Just remember that intelligence is relative, a zombie as intelligent as a dog would be much more deadly than your run of the mill zombie but not equatable with human intelligence.
If the zombie has some logic but no long term memory then weapons would be out. They could in theory climb ladders, work simple puzzles, fumble with door knobs but not understand tools. What you are describing is definitely not a zombie by any reference I know of. It would more closely resemble a Lich. However, as with all mythical or contrived creatures it doesn't rule it out.
Lets say your virus that creates zombies not only keeps the flesh animated and destroys the entire higher brain functions but instead only affects the frontal lobes of the brain. Thus your zombie virus has roughly the same effect as a frontal lobotomy. It would have to attack various other structures of the brain so that you could get the required effect. I would look at what is needed and what is contraindicated and look at what areas of the brain are responsible for each and construct my virus pathology to match the list.
Minor Logic (stick hit good)
Higher Logic (math)
Maybe someone who is into congnative psychology or neurology could help you distill it down so you know what areas of the brain are effected and which are not and possibly even why they are affected (common hormone or chemical present in the needed or not needed areas).
Make the virus paralyze/disable their hands. No hands, no effective way of using hand-held tools/weapons. And it's not easy to stab someone running away from you with a foot.
OR: They chew their fingers off. The virus has them gnaw on their hands, malforming them and making them unable to wield tools as they have no finger dexterity.
Building on socrates's answer.
Have them hunger for not only fresh flesh, but even the "dead flesh". In their initial hunger they would've the only logical choice... gnawing of their limbs.
This could also lead to interesting side consequences for the "dead" Zombie genre.
- What happens to the sheer number of zombie groups, Zombies are intelligent but they can't be in groups lest they each other. Depending on the intelligence level they can maybe create small groups.
- Animals. ALWAYS fear animal zombies.. Always. That one dog you wanted to pet.
- So I think this makes us into some Zombie x Vampire territory. Instead of vicious hunger for blood, they hunger for flesh. Human flesh is the best but animal or other Zombies MAY suffice. Worst case scenario... Eat yourself.
Give them the shakes! Constant uncontrollable tremors in the hands and arms would make any firearm useless and might even turn the weapon into a tool for self mutilation and destruction. If a trigger finger twitches while the barrel is pointed at your foot, then bye, bye foot!
Since your zombies are intelligent, they will learn not to pick up guns after watching a few of their walking-undead brethren become crawling-undead.