In a multiplayer game, I'm playing Exquisite Blood and resolve the second ability of Aetherflux Reservoir, dealing 50 damage to an opponent who's life total was only 10. I was able to pay the 50 life - that's not part of the question.
What I want to check is if, when Aetherflux Reservoir's ability deals 50 damage to my opponent, Exquisite Blood's ability will make me gain 50 life, or if I only gain 10 - because that was my opponent's "available life" before.
Rule 119.2 says “Damage dealt to a player normally causes that player to lose that much life”, and Rule 119.6 says “If a player has 0 or less life, that player loses the game as a state-based action”. The “or less life” makes me think that a player's life can be negative, and so I would gain 50 life - but I may be missing any other relevant rule here.
I know I can't pay more life than I have per rule 118.4. But that doesn't tell me anything regarding losing more lives than I have.
(not sure if tagging mtg-multiplayer since this can happen in 1v1, but can't think on a situation in which it matters)