I played Haven's last stand this evening and the antiheroes found themselves at a point where they had no further characters in reserve. The avatar was the final marked monster. There was a bit of a scrap and both the avatar and a hero died simultaneously a result of a fight.
Was the game a draw? Both sides achieved their win conditions at the same time.
Paul G is correct... page 13 from the rulebook: "Ties: If both victory conditions take place at the same time (the last Mission is accomplished but by doing that a Hero is killed and there are no more in reserve), victory goes to the Hero Players."