The same kind of thing happens when a voodoo priest poisons some poor sucker, dumps him in the grave and recovers a brain dead slave a few days later. These things can endure trauma that a normal human would never be able to take because most of them can't feel any pain. Luckily, they don't have the kind of endurance that reanimated zombies do. Oh, and make sure that you don't take any of what the zombie did. These are the things that give everyone the creeps.
Dead people are one thing.
Ghosts are one thing. Brainless dead things that like to eat people and refuse to die are quite another. As a result of some chemical or magical event these walking corpses have returned to life. Although they aren't actually alive. Frankly, we don't have any idea what they are.