Zombie means “living dead,” and was originally a god in different countries in Africa, particularly in the Congo. However, the term also grew into an integral part of voodoo magic describing a corpse that has been enchanted and brought back to life by having part of its soul restored by a sorcerer, who then has control over the zombie.
It was believed the only people who can become zombies are those who have had an unnatural death. Thus to prevent the sorcerer from being able to bring them back as a zombie, was to symbolically kill them for a 2nd time.
Of course Hollywood has now added several incidents where one might get zombied.