Killed while incapacitated by the BBEG. What do you think?

Quote from Jammer_Ninja >>

How would you feel?

 Bad, especially when invested in the campaign. While playing a D&D character should come with the expectation that it can always die, there's ways to dying that feel more cheap than others, and mass-attacking unconcious PC is one of them even if justified somehow. 

As DM, its very rare i'll have an enemy target unconcious character, unless its very evil or voracious, most will focus their attention on other combattants still posing a threat instead.

Quote from Caerwyn_Glyndwr >>

Quote from HolyAvengerPlusFive>>

Having NPCs target incapacitated PCs during a fight is very immersion breaking because it suggests the NPCs have an awareness of the DnD game mechanics, and don't want a collapsed and defeated character to succeed their death saves.

What you call “immersion breaking” is likely more accurate to reality than “I see someone go down and just assume they’re out of the picture, and do not make sure the job is done.”  Particularly true in a land where magical healing exists.

Take, for example, real medieval combat.  When fighting a knight, the entire goal was to knock them down, perhaps incapacitate them with blunt weapons… then walk over and finish them off.  There was even specialized equipment built for this purpose, like the French misericorde (a thin dagger that could be slid between the eyes of a visor or other gaps in armor to deal fatal blows).

Or combat with wild beasts, who might continue attacking a downed body until signs of life have left it.

There are reasons a DM might not attack a downed character - “immersion breaking” is not one of them, and, frankly, reality suggests it is more immersion breaking not to have enemies try to finish characters off every now and then. 

 
I would interpret your example as an NPC knight-killer looking to perform the knockdown effect on a knight, with tools used on these knights once they are knocked down. Stabbing a knight through the visor after a knockdown and reducing them to 0 hp, blood running out from the knight's visor as they stop moving, would then be the incapacitated status, imo. Having the knight-killer NPC then begin ducking and dodging other opponent attacks while kneeling down on the ground to spend multiple rounds stabbing the unmoving knight over and over would then be the equivalent of multiple attacks on the incapacitated knight.

From my perspective, the story told from stabbing a knight in the face, having them stop moving while blood flows out of their helmet, is enough to say that the knight was defeated. Of course having a game with magical healing where PCs can heal, creates a feeling from the DnD mechanics side as a _game observer_ that you need to "really kill" the PC to avoid them being healed, but the incapacitated rule exists for PCs because they're the main characters of the story. The NPCs don't know they live in a tabletop game where they don't get death saves but the PCs do. Having NPCs treat the PCs like magical trolls or vampires with natural regeneration just because the targets are PCs is where the immersion breaking part comes in. If an NPC brings another NPC to 0 hp and that target NPC immediately dies, does the NPC that brought the target NPC to 0 hp start spending multiple turns hacking the body to pieces despite the body just being a physical object at this point? No? Then why not?