Told my friend about this thread and we came up with what we thought was a pretty cool concept.
Mindless undead always default to killing the living around them, but are otherwise just tools of a Necromancer.
Intelligent undead are generally evil but exceptions may exist if they chose to be undead as guardian spirits or Liches or something like that.
Using magic wears holes between the Material plane and all of the others. However as most magic is transient these holes are so quickly repaired and so small they have no effect (a fireball lasts about a second.) However Undead are the exception to this as they are both common and very long lasting. Lots of Undead in an area being created created leaks to the Negative energy plane which initially cause spontaneous Undead creation but eventually lead to full out portals to the Negative energy plane.
Now there is a nation whose entire unskilled workforce is basically Undead. Their entire economy depends on these Undead and very rich companies lead by liches make their fortune off raising Undead. Naturally this eventually starts creating large holes to the Negative energy plane and masses of uncontrolled Undead. Eco-terrorists have begun attacking mines and killing all the zombie workers claiming it's the fault of Undead. The Liches naturally claim that this is pure bogus and point out that the country needs it's Undead. The PCs have to decide what to do in this situation.