It is disingenous to bring up RAW to defend part of your hypothesis and then ignore RAW for another part of it.
I agree with this.
To make this work a single stab from a dagger needs to be able to kill a 20th level PC and it needs to be able to kill them in a statistically significant fashion.
If it can't kill you then there is no logical reason to fear it.
I agree with this too.
In the example that originally started this tangent - a person ignoring the warrior attacking them in melee in order to strike at a dangerous foe 30' away - I don't see any difficulty reconciling that with the hp mechanics. The person is being attacked by the warrior. They are parrying/dodging/deflecting those blows, which thus do only glancing damage at best - the mechanics that produce this fiction are the AC rule and the hp rules. Meanwhile, the person focuses most of their attention and effort on the dangerous foe further away. We
know that this isn't suicidal for the person in question, because the AC and hp rules tell us as much.
In my imagine, Conan can certainly hold off a couple of bandits, or even knights, while he throws a knife at a sorcerer standing on the other side of the room! It's no surprise that characters in D&D are similar in respect of what they can do.
IRL I am not terribly afraid of being stung by a yellow jacket and that is probably comparable to what a 20th level PC probably feels from someone shoving a dagger into his chest. Do it 50 times or so in a minute and I might just die from being stung, just like that 20th level PC might just die from 50 stab wounds.
I have a different view from this, though. I find the idea of high level D&D PCs as being pin-cushioned in arrows, or being stabbed multiple times through their vital organs, and yet continuing on unhindered, a bit too silly for my taste.
I prefer the approach that Gygax sets out (with a modest degree of clarity) in his DMG: hit point loss is a game mechanic that tracks how badly the injured character is being set back in their endeavours; but how that "being set back" is
narrated can be highly flexible.
Putting all the above together, I therefore disagree with this:
It's just that once you go to narrative stuff like "hero murdered in his sleep with a dagger to the heart," then you need to just say "anyone could be killed if they're helpless in a non-combat situation if the narrative calls for it," and the rules don't cover that, excepting for the "what the DM says, goes" part.
Once a PC has reached mid-level, if they are at full hp then they typically can't be murdered in their sleep. In mechanical terms, they take (say) 8 hp of damage from their (say) 40 hp remaining. In the fiction, they wake at the last possible instance and roll out of the way, and so suffer only a glancing wound (or graze, or whatever it makes sense to narrate). As a result of this they are somewhat set back (as indicated by having only 32 of 40 hp remaining). But they are not stabbed through the heart!
For the GM to exercise fiat and just declare the PC dead; or introduce a different resolution mechanic; would be - in my view - a pretty controversial attempt to alter the rules of the game on the fly!
I think the discussion comes from people trying to simulate every event in the world with the game's rules. "How can the warrior-king (an NPC) have been killed by a single dagger stab while asleep?! His traitorous wife with a dagger does 1d4+1 damage and the king had 144hp!"
NPCs are a different matter, in my view. As a general rule (eg unless we're talking about a NPC who is important to a player's position - like, say, a family member or friend), NPCs are the domain of the GM, and so the GM can't be unfair to the players by narrating things like deadly stabbings in the night.
The only concern here is verisimilitude - eg if this king is so puissant as to be able to (say) hold his own against the 12th level PC fighter, how was an ordinary person of merely ordinary strength and skill able to stab him?
Also hit points are an abstraction and your thematic description of what hit points represent (luck, skill, blessings) is fine, but it is one of many possible thematic explanations. I could just as easily say hit points are due to thicker skin and more robust or even multiple organs offering combat redundancy and that "explanation" is every bit as valid as yours. Further in a world with magic there are numerous things that require even more suspension of disbelief than this explanation would.
I strongly prefer the Gygaxian approach - with hp loss corresponding mostly to ways of being set back
other than brutal physical injury; and with a PC's high hp being a reflection of their skill, luck, divine protection and the like - to the alternative that you suggest. I find your alternative a bit too bizarre.
I am still waiting to find out why someone who gets stabbed by a poison weapon, or clawed by a Quasit or similar can have the poisoned condition when the weapon or other attack completely misses them and causes no actual damage.
Gygax discussed this in his DMG. Those attacks get narrated as pin-pricks from the blade, or quasit's tail, or whatever, that inflict little physical injury but are sufficient to introduce the poison into the PC's bloodstream.
Morover, there is nothing in here about being willing. In the case we were talking about (being suicidal). just because I demonstrate no regard or concern for my life does not mean the divine intervention or luck or whatever doesn't continue on.
If we are talking RAW laying down and letting someone stab you in the heart will not kill you.
This is a good point: the gods may not want the character to die; and so prompt some sort of response, or deflect the blade, or whatever.
