The questline is one quest following onto another, so problems in one quest will cause no other quest to be doable or even receiveable.
Unfortunately the townguards can kill the questmobs before actually knowing about the quest if pulled, which happens from time to time cause of the huge aggro-range some creeps evolve. For example the 'bandit-mobs' can follow you up to the necrocity - without being hit by you more then once. The killing of mobs should require more effort than trying to defend them from the guards, there should not be the need to suicide only to prevent the guards from killing the (quest-)mobs.
Possibile Solution:
- Put the guards inside the cities and create a creepkillzone, connected to respawns of the killed mobs, in front of every gate.