Additions:
As an Undead being, there is something innately unnatural about Zombies. In their case, it is more of an echo of something wrong. Either way, their continued existence brings about destruction.
Zombies
The reanimated corpses of those deceased. Incapable of even basic reasoning, Zombies are amongst the most basic of Undead. If left on their own, Zombies will actively seek out and destroy any life they are somehow able to sense nearby. Over time, they will generally rot away until they cease to function.
Generally, they are not much of a threat on their own. Even in large hordes, they lack any real sense of tactics. However, if there is a large zombie horde, it is most likely being controlled by some intelligence.
Zombies are innately magical creatures. Lacking a will of their own, they move by the power of others. This may be a deliberate act such as being created through Necromantic Rituals, or as some unintended side affect. Ghosts in particular have been known to be able to create zombies.
Strangely, uncontrolled Zombies will never attack anything but the living. Devils, Demons, Spirits, and other Undead are generally safe. If ordered to attack, they will, however without nearly the same amount of zeal.