How many important tasks could you achieve on this planet if you killed everyone except yourself?
ALL OF THEM. Kill one man, and you are a murderer. Kill millions, and you are a conqueror. Kill them all, and you are a god! What could possibly be more important than being a motherfucking GOD?
if the task is being a god ¯\_(ツ)_/¯
not to mention you'd be a god to yourself alone, with powers that are now useless (because there's no one left to kill). Boyo, you could have had it easier if you just wanted to entertained that delusion.
Then the main quests should just be designed in a more modular way, rather than the typical "you need to tackle this sequence of quests in order" affairs.
like I said, it's often about following a trail of information. If you stumble on something going on right away, it's useless for you to go back and do another quest that would have led to information to pinpoint you to that other place. If you get access to some place on your own, it's useless to go back to some other quest that would have allowed you to get in there by some other means.
So killing NPC's left and right could be a tactic to brute force your way into places you'd otherwise get no access to. Not a problem with that. It happened regularly that way with Fallout or Arcanum. Thing is, if you brute force your way inside some location, there shouldn't necessarily be a way to do the same things as you could have done if they had let you enter willingly. And it's absolutely fine if killing NPC's in one place closes off other places to you as well.
We can assume information travels in the game world independently of you. I think that's one of the things whiners can't wrap their heads around, they believe if they left no witnesses, then what they did in one location should never affect another location. Except by the game's lore, those places aren't shut off from the rest of the world. Just because
you don't see NPC's travelling between locations, doesn't mean it's not happening by the game's logic.
Of course, there's no need to make the main quest completely unfailable. If you dick around too much and purposely break quests, you might get stuck in an unwinnable state, yes.
But putting in alternative approaches for finishing the game is always a good thing and a sign of good game design.
Again, Morrowind is a great example for it. You have one failsafe alternative solution if you kill, accidentally or purposely, NPCs important to the main quest. And even that one can fail if you kill certain characters (IIRC you can't kill Yagrum Bagarn or you're screwed entirely).
Alternative solutions are gud, we know them from the real world. OTOH, killing people you have need of is one of the biggest fuckups in reality too.
Maybe you could say the "problem" in games is that people seem irreplacable too often; if one NPC has privileged information due to their status in some hierarchy, and I kill that guy, there should be a successor who's going to have access to the same information. Unless the information wasn't put down in writing but supposed to be bestowed by old guy on new guy. You'd be opening a whole new can of worms that way though, because how is new guy going to react to you if word got around that you killed his predecessor?
Arcanum tends to supply the most alternatives for its main quest:
- finding out who the ring belongs to leads you to P. Schuyler and Sons; they use dwarven zombies as slave laborers and if you have Magnus in your party he gets super angry and wants to kill the bastards; you need information from them though; you can talk to them to get the information, which will piss Magnus off if you have him; you can kill them all, then use conjure spirit to bring the spirit of their father back from the relam of the dead to give you the information you need; you can rifle through their archives to find a document containing the information you need
- you usually work for Gilbert Bates, but if you kill the guy the main quest isn't busted: you can still work for his rival Cedric Appleby, offering you an alternative
- when you need to travel to the Isle of Despair you gotta get a boat; now, I'm not 100% sure if this approach works since I haven't tried it myself yet, but if you kill the captain of the boat that is supposed to bring you there, you might be able to get there by acquiring the ship of ghost pirate Stringy Pete
I actually liked how Arcanum supposedly let you resurrect anyone. Supposedly.
The problem becomes evident at the crash site:
So you talk to some ghost that just randomly appears. You want information from the guy, but how are you going to get that? Friendly Ghost delivers himself into your hands by saying that he won't find rest until you've avenged him. It's not going to be that easy every time, or is it?
Fast forward to Shrouded Hills, you kill a weak NPC (like the Herbalist), resurrect them... nothing. No dialogue. Even though that NPC later turns out to have had vital information for the main quest.
There are always some alternatives. Services that are required to progress might be offered by different people. Information held by people might be contained in documents or wall inscriptions. Etc.
how many times in this world have you found a vital piece of information written on a wall?
That's exactly the problem, alternative solutions may exist, or they may not exist. And when they seem to obvious it gets cheesy.