Sadly a lot of Animation and Target Classification thingies are hidden deep in the code of WK3. If you want the Death Coil to dmg enemy undead units, then modify all undead units' classification not to be undead. If this is an RPG map then do this with the undead units you want to be the target of the spell.
If you only want to make a damage dealing spell then use another spell. If you want it to retain the healing effect, and still damage enemy undead units of the same type of urs well.. thats' impossible.