When i think of undead i think of creatures you can hit much much much, but they will allways stand up. So, for new undead creatures or even the old ones. When you 'kill' then their body will fall on the floor. Then after 5-10 min the body stands up again and fights. Perhaps only the turn of pally/cleric can really kill them.
text example:
You see a Zombie
k zom
You see a zombie corpse
(5-10 min)
The zombies stands up again
You see a Zombie