I just wanted to know what happens to an NPC if you killed them or they got killed in a random pub fight... Do they get replaced or can you just kill everyone until your the last one alive? (PS3 version)

NPCs generally fit into four categories -

-The largest group are named NPCs that, when they die, are gone for good.
-The second group are called 'essential' because they can be knocked unconscious, but not killed. These can be identified when you approach them becasue they have a crown type symbol instead of a round one when you are close to them. These NPCs are normally essential to support one quest or another. Sometimes, their status changes to nonessential after their 'role' is completed.
-The next group are what I call generic or unnamed NPCs - like Imperial Legion Soldier, Imperial Forester, Courier, Bravil Guard. These NPCs can be killed, but respawn after three days.
-The final group is a very small one. These are named NPCs that can be killed but will respawn.

For most NPC's dead is dead. No matter if that's by your hand or random. A few NPCs are replaced by another, completely different character. Those rare occasions happen only when the first NPC falls to a scripted death.

You can't kill everybody. The guards will respawn. There are a few characters marked essential. They will just be knocked out for a few seconds. Everybody else is fair game. Although, you mat be killing future quests as well.

Isn't there only one?

Cause I killed one of the Draconis family member, and I was planning to do the Datk Brotherhood. So I wasn't sure if I could still do the quest when I already killed one of the targets...

Lothran is quite happy that Ongar belongs to that group

I can think of a handful of named NPC's that respawn. To avoid spoilers, let me offer this link that lists both named and unnamed respawning NPCs: http://www.uesp.net/wiki/Oblivion:Respawning_NPCs


That will not cause you a problem with the quest.