My Secret Santa is the best!
Also if there is only one monster alive, the game ends immediately.
The 2E rules stat that "You must survive your turn to win". So a monster could for example be poisoned, but not roll enough hearts (or be in Tokyo) to survive. In that case everyone loses.
However, the rules are not clear regarding this, so I could be wrong.
But this is anyway a very rare situation. It has never happened in any of the games that I have played.