Actually, in 1E and 2E, 0 hp = death. It was optional, but normally highly recommended, to allow PCs to go to -10 before they died, losing 1 hp / round IIRC.
I could be wrong, but didn't 2e bake in the death-at-minus-10 rule?
In Basic and 0e death was at flat 0. 1e defaulted to death at 0 but Gygax offered a few alternatives, of which by far the most-used was death at -10.
The section on hp recovery (same page):
View attachment 114823
Yeah, 1e hit point recovery as written is too slow even for me; particularly when most of one's hit points are supposed to be luck, nicks, fatigue, etc. that should in theory come back fairly quickly (though not all in one night!)
It just makes a case for a body point/fatigue point split, is all.
Oofta said:
I really disliked older editions that had death at -10 and house-ruled it when I ran my own games. Dying at a flat -10 was so arbitrary. What was so magical about that number? If I had two PCs, one with 5 HP and one with 100, the former pc for all intents and purposes had double their HP as a "buffer" before dying. A low level PC will be fighting monsters that will rarely kill them outright in one blow, even if they were already wounded. High level PCs? Get low on HP and any single attack could take you from perfectly fine to flat out dead.
The reason for the flat death-at-minus-10 was, as far as I can tell, to try to reflect that when it comes right down to it, each of our physical bodies can take about the same amount of actual wear and tear before falling apart; and once you were into negatives that bit of physical resilience was all you had left going for you.
It's yet another argument for a body-fatigue system. Negative points, and the first few positive points, are body points: harder to recover or cure and (once in negatives) giving significant penalties to what one can do and how effectively one can do it.
For what it's worth, I extend the same courtesy to most living monsters, villains and opponents: they too die at a minus number vaguely determined by their size and type: a simple Kobold might die at -6 while you might have to take a Cloud Giant or massive Dragon down to -20 or -25 before finally finishing it off. The default is -10. Undead and some jellies and oozes are destroyed at 0.
That said, all creatures suffer penalties to just about everything if below 0, and also have to make checks to remain conscious.