Oh for Almsivi's sake not this again.
1: The main quest had several outs even if you killed a main quest important NPC which is what lets people speedrun the main quets in about 10 mins.
http://www.uesp.net/wiki/Morrowind:Yagrum_Bagarn_and_Wraithguard
http://www.uesp.net/wiki/Morrowind:Hortator_and_Nerevarine
2 remember Din?
http://www.uesp.net/wiki/Morrowind:Din
The reason he respawns is the same reason why later games has the essential tag, NPC's can be killed outside of player agency, by animals, other NPC's or falling to their death because of awful pathing.
How convenient that people so often forget to mention these argument undermining facts.
Add to the fact that post morrowind NPC's could move about, travel and could easily get killed en route with no player interaction whatsoever thus making the essential tag a necessary mechanic to prevent unecessary frustration and annoyance.
In one of my Skyrim runs I held off doing a quest for a long time and then when I tried to do it I couldn't find this guy:
http://www.uesp.net/wiki/Skyrim:Tacitus_Sallustius
Found his remains in the Halls of the dead.
Turns out he had died without the Dovahkiin murderising him, no idea how, probably dragon attack or more likely falling to his death in a moronic way.
A level 6 who didn't have the essentail tag and so no quest, no smithing skill up. So sad.
Dawnguard vampire attacks, no more needs to be said.