They support the evil and are against the good and everyone alive.
The revived dead who used to be people, orcs, taurens, etc. during their lifetime. They support the evil and are against the good and everyone alive. They coexist with the Horde because the representatives of the Alliance serve the Light which is a solid reason for the Undead to destroy them. In general, motivation of the Undead often changes and depends directly on their leader who changes quite often as well.