FormerlyHemlock
Hero
OK, I follow your math of how death occurred in both A and B, just a matter of which hit was a crit is the difference between 3-4 fails. But I was erroneously focusing on the grand total of damage = instant death, not "any subsequent damage taken is a death fail, and a crit is two."
Note: keeping track of negative HP is a somewhat common house rule for DMs who dislike the effects of "can't go below 0 HP." If you like keeping track of grand total damage, go ahead.
If you did this, you could just say e.g. each failed death save costs you 20% of your total HP (round down, and crit fail costs you 40%), so you'd need five failed death saves to actually die from something that barely took you down to exactly 0 HP, but you could die from as few as one failed death save if the thing that took you down almost insta-killed you. In this case that would indeed mean that the Barbarian went from -64 HP to -92 HP (71 * 0.4 = 28, 64 + 28 = 92) which means he is dead after his first failed death save. Same result, but different math.
The main consequence of doing this would be to bring back an old school vibe to combat, and make it less easy for someone to soak multiple gigantic breath weapons from Tiamat and then be back up on their feat after getting just 1 HP of healing from the Healer feat or Healing Word or Regenerate.
Hmmm, now that I've written this idea down I am going to have to try it...