The two examples I can name off the top of my head are from The 13th Warrior when Buliwyf, dying from the mother's poison, fights his final battle and wins regardless, and in David Gemmell's Drenai Cycle when Druss fights his final defense of Dros Delnoch after being poisoned in an earlier duel, managing to die of his wounds rather than that poison.
I'm sure there are any number of other examples; I'm wondering what the first use of this particular version of the hero's last stand was (in fantasy or science fiction)?
Important note the hero in these cases is poisoned by the people he's fighting.