Without medical treatment, a character with negative hit points will die. Having negative hit points is a fatal condition indicating overall systemic failure due to injuries.
Without medical intervention, a character must, each round, make an Endurance check at -1 for each hit point below zero. If the check is successful, the character remains at the current hit point level. If the check is failed, then the character loses another hit point from either internal or external bleeding. A wounded character can remain alive even down to -10 hit points. But once the character's hit points fall to -11, the character dies, and only a Wish, Raise Dead, or similar power can bring the character back from the dead.