Let me get this straight.

NPC's who are part of quest-chains (Mainquest, Companions, Thieve's Guild, etc) are invincible, but may be subdued.
NPC's who are part of minor quests (like Lucan in the Golden Claw quest) are also invincible, but may be subdued.
NPC's who are part of a miscellaneous quest can be killed permanently.
NPC's who are not part of anything may be killed permanently.

Is that correct? So in this sense, I can go around the whole of Skyrim and slaughter each and every individual except the ones who offer quests (minus the miscellaneous ones)? So in a way, it is impossible to miss out on quests cause that NPC can never die, correct? You can only miss out on the miscellaneous quests. What if I finish a quest-chain, such as the Golden Claw, will that NPC (Lucan) be vulnerable then?

Reason I ask is cause I slaughtered everyone I could in Riverwood and Whiterun, but I am on a mission to slaughter every possible being in Skyrim and become the the ultimate Vampire Lord.. I know it's a bad idea, but it's to late to stop now.

Bump

Nope, you can still kill enough people to mess up quests, just not enough. For instance you can kill Brand-Shei and so never do his quest. Most of the NPCs who are essential lose the essential tag at some point.

I believe you are basically correct, although as Abrinth points out you can mess up various quests by killing non-essential NPCs that are associated with them. Ysolda, for example. Anyway, check http://www.uesp.net/wiki/Skyrim:Essential_NPCs, although it may be incomplete. Doesn't include Lucan, although his character page says he's essential until after Golden Claw. I can confirm he's killable after that.

Thanks guys.

Isn't Blood On The Ice another infamous example where you can kill NPCs beforehand, causing it to break the quest?

I believe this one is incorrect (as others have pointed out).

I could not kill Lucan, and he's part of the Golden Claw quest, which is a minor quest.

I meant in general. As others have mentioned if you around trying to kill every NPC you see you will f up some non-MQ quests.

we all play differently. I doubt I could kill everyone. just not that violent. My characters can be but they also have a conscience lol.